View source for Book - Russian Embryology (1750 - 1850) 25

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==Chapter 25. Investigations on Invertebrate Embryology - Work of A. Grube, A. D. Nordmann, N. A. Warnek, and A. Krohn==


During the first twenty to thirty years of the nineteenth 
century, embryology remained chiefly the study of the embryonic 
development of vertebrates; the comparative peculiarities 
of development of different animals was studied only at the 
limits of this most studied group. By the end of the twentieth 
year of that century, i.e. the period of Baer's active work 
in embryology, the first investigations into the development 
of invertebrates appeared. Baer himself, as mentioned above, 1 
turned his attention to the characteristic peculiarities of 
development of arthropods, noting in particular that their 
blastoderms are situated on the abdominal side of the egg and 
distributed from here in the dorsal direction. Baer was not 
able to explain the development of arthropods more clearly. 
Even the investigations of his predecessor Herold, which were 
limited to the study of the late stages of development of 
butterflies and spiders, 2 could not give Baer material for 
well-grounded comparative embryological conclusions. 

The aspiration to apply embryological principles to 
arthropod development was actualized by Baer, first in the 
study of embryonic layers, reflected in the works of 



1. See Chapter 15. 

2. J. m. D. Herold, ENTWICKELUNGSGESCHICHTE DER SCHMETTERLINGE ANATOMISCH UND PHYSIOLOGISCH BEARBEITET 

(Cassel u. Marburg, 1815), vi + 118 + xxxiv pp.; EXERCITATIONES DE ANIMALIUM VERTEBRIS CARENTIUM IN 0V0 
FORMATIONE. DE GENERATIONE ARANEORUM IN 0V0 

(Marburg, 1824) , x + 63 pp. 


M. H. Rathke. A native of Danzig, Rathke in 1829 arrived 
in Russia, where he was professor for six years at Dorpat 
University. Before that, he published many embryo logical works, 
including a valuable work on the development of crayfish. 3 
During his tenure at Dorpat, Rathke visited Moscow and 
Petersburg and also travelled in the Crimea to investigate 
the fauna of the Black Sea. On the Black Sea coastline, 
Rathke collected comparative embryological material which 
was later used in ON MORPHOLOGY: TRAVEL NOTES FROM 
TAURIA.4 in this collection, in addition to brief information 
about the embryonic stage of actinia, there is also an essay 
on the embryology of the Crimean scorpion and investigations 
concerning the development of nine species of Crustacea of 
different orders (copepods, araphipods, decapods, and isopods) . 
Rathke 1 s work on crayfish development, and also his investigations on the development of other arthropods (124), represent 
a clear interest in describing the phenomenon of embryonic 
development in arthropods according to the ideas of Pander and 
Baer. Rathke spoke of the embryonic disk or blastoderm, of 
the primary cavity, and of the two embryonic membranes (serous 
and mucous) into which the blastoderm is divided. The first 
stages of development, the division of the ovum and the first 
processes of separation of the rudiments, remained untraced. 

It must be borne in mind that a clear presentation about 
the essence of the processes which take place in the early 
stages of embryonic development â€” i.e. first of all the process 
of division â€” was not yet established in the first forty years 
of the last century. Thus, Reichert (125) studied the development of the frog ovum but reached an incorrect conclusion 
about the structure of the still-undivided ovum, supposing 
that it consisted of many cells (by cells he meant the round 
accumulations of the yolk plates) . This point of view was 
raised by Reichert both in his DIE ENTWICKELUNGSLEBEN 
IM WIRBELTHIRREICH and in an article published one year 
before, "On the Process of Division in the Ova of Amphibia, "5 



3. H. Rathke, UBER DIE BILDUNG UND ENTWICKELUNG DES 
FLUSSKREBSES (Leipzig, 1829) , 97 pp. 

4. Rathke, ZUR MORPHOLOGIE. REISEBEMERKUNGEN AUS TAURIA 
(Riga und Leipzig, 1827) , 192 pp. 

5. K. B. Reichert, DIE ENTWICKELUNGSLEBEN IM WIRBELTHIERREICH (Berlin, 1840), x + 261 pp.? "Uber den Furchungsprozes 
der Batrachier," ARCH. ANAT . PHYSIOL. (1841), pp. 523 541. 

514 



in which he wrote: The process of division of the amphibian 
ova is nothing more than successively accomplished generic 
action (GEBURTSACT) of the maternal cells, repeatedly 
invested in each other. 

T. Bischoff, working on the embryology of mammals and 
publishing in the period from 1842 to 1852 a monograph 
about the development of man, rabbit, dog and guinea pig, 
also did not reach a clear understanding of processes of 
division and did not recognize the spheres resulting from 
division as cells; since in his opinion, the cell must 
possess a cavity, but the spheres resulting from division 
are filled with yolk, the nuclei of the blastoderms were 
taken by Bischoff as fat droplets. 

A. K8 Hiker** went much further in the analysis of the 
process of ovum division, admitting the direct continuity 
of the blastoderms and those cells from which the embryo is 
built in later stages. 

In all these cases the discussion was about the forms of 
development, complicated by the great quantity of yolk in 
the centrolecithal and telolecithal ova. The nature of the 
occurrence there of superficial and discoidal division was 
explained much later, after the introduction of sectioning 
in embryology. 

A more distinct presentation on the phenomena of 
division was stated by Baer in his work on the development 
of amphibia, 7 but especially in the work noticed by his 
contemporaries and later forgotten on the development of the 
ova of the sea urchin. 8 

Certain embryologists of the thirties and the forties 
nearly approached the correct interpretation of the phenomena 
of the ovum division. They include KClliker, then Loven,9 



6. A. KSlliker, ENTWICKELUNGSGESCHICHTE DER CEPHALOPODEN (Zurich, 1844), 180 pp. 

7. See Chapter 21 . 

8. See Chapter 23. 

9. S. L. Loven, "Bidrag til kannedomen af Molluskenas 
untveckling," K. VET. AKAD . HANDLINGAR (1839), 
pp. 227 - 241. 


Sars,10 van Beneden,H and Quatrefages-^ and must be 
mentioned. The first four investigated mollusc development, 
and the last studied the development of annelids. They 
presented some stages of ovum division, but did not broach 
the subject of the internal processes occurring in it nor 
of the fate of the spheres of division. 

A great part of the works of that time concerning the 
development of invertebrates was illustrated by the study of 
different types of reproduction, and also by the description 
of the structure and the transformation of different larval 
forms which sometimes did not yield to systematic determination and figured under different specific names (126) . The 
investigations of forms of reproduction became especially 
popular after Steenstrup showed the wide distribution of 
the alternation of sexual and asexual generations of many 
invertebrates; the application of this empirical regularity 
to groups of animals not investigated before this time 
constituted the majority of works in the present sphere. 
The single base for generalization concerning early embryonic 
development is thought to be the cellular theory formed 
shortly before this. Using extremely imperfect microscopic 
techniques, the embryologists of the first half of the 
nineteenth century posed the question, can the spheres of 
division be called cells, and are the cells of which the 
embryo consists the direct descendants of the primary 
spheres of division? They attempted to trace the fate of 
those existing in the unfertilized ova "embryonic vesicle" 
(and "embryonic speck"), i.e. to explain whether these formations disappear after fertilization without a trace or 
whether they stand in continuous genetic connection with 
the nucleus (and the nucleolus) of the cells of the embryo. 
On the foundation of a one-sided and primitive understanding 
of the cellular theory, fantastic presentations sometimes 
grew, like the ideas of Reichert, which resurrected the 
long-buried theory of preformation. 



10. M. Sars, "Beitrag zur Entwickelungsgeschichte der 
Mollusken und Zoophy ten," ARCH. NATURG . , 6 (1840), 
pp. 196 - 219. 

11. P. J. van Beneden et Ch. Windischmann, "Recherches sur 
l'embryogenie des Limaces," ARCH. ANAT . PHYSIOL. 
(1841) , pp. 176 - 195. 

12. A. de Quatrefages, "Sur l'embryogenie des Annelides," 
ANN. SC. NAT., 3 Ser. Zool. 8 (1847), pp. 99 - 102. 



For the purposeful coordination of the efforts of 
zoologists studying invertebrate development, the theory of 
embryonic layers, which still had not become a broad 
scientific generalization, could not serve. Its correctness 
was proven only for vertebrates, but the application of 
the theory of embryonic layers to invertebrates was believed 
by nearly no one. Rathke's old data concerning crayfish were 
known. More than a quarter of a century later Zaddach also 
reported on the embryonic layers of insects, admitting to 
his descriptions crude morphological mistakes. 

The data related to the development of different types 
of invertebrates were accumulated relatively slowly, because 
a theoretical conception for which this material could be 
used did not exist. The study of the types of structure and 
development of animals, taking its beginning from the opinion 
of Cuvier and Baer, fell into decay. The formally opposite 
theory of types, that of Naturphilosophie, the idea of 
unity of planes, caused little enthusiasm. Only after fifty 
years, after the appearance of Darwin's theory, did this 
period of mental stagnation end. Then the naturalists were 
divided into two camps â€” the hearty supporters and the violent 
opponents of the theory of evolution. Practically none of the 
contemporaries of the great reformer of biological science 
could keep an olympian calm and sustain neutrality for any 
length of time. 

The investigations preceding the publications of the 
ORIGIN OF SPECIES played, however, its historical role. 
Factual material was collected which Darwin and his early 
followers used. In this period came the important works of 
the Russian embryologists A. Grube, A. D. Nordmann, 
N. A. Warnek, and A. Krohn. 

To Dorpat University professor A. Grube belonged serious 
investigation on the development of annelids; he studied 
the embryonic and partly post -embryonic development of the 
Proboscidea leeches Clepsine oomplanata and C. biooulata 
(genus Glossosiphonia, according to the current terminology). 13 

Adolf Edward Grube* 4 was born in 1812 in Konigsberg, 
where he graduated from the university. For thirteen years 
C1843 - 1865) he was professor of zoology and comparative 
anatomy in Dorpat University, and to this period of Grube's 
life are related his most important works in systematics, 
anatomy, and embryology of the annelids. 

Grube's work undoubtedly possesses remarkable significance. Because it was undeservedly forgotten and not 
mentioned even in the detailed reports and special works on 
the development of leeches (127), we must dwell here on its 
content. 

In the introduction to his work, Grube turned to 
memories of his years of study in Konigsberg, where Baer 
was working. Actually the great embryologist at that time 
read few lectures, being engrossed in the investigations 
of development of fish and amphibia. He was helped by 
Grube's friend, the clever graphic artist Burov, who, by 
the way, encouraged Grube's attraction to the laboratory, 
where the latter acquired a taste for embryological 
investigations. Grube was aware of the difficulty of these 
observations, but he did not abandon his idea of devoting 
himself to embryology. 



13. A. E. Grube, UNTERSUCHUNGEN UBER DIE ENTWICKLUNG 
DER ANNELIDEN. I: UNTERSUCHUNGEN UBER DIE 
ENTWICKLUNG DER CLEPSINEN (Konigsberg, 1844), 

ii + 56 pp. 

14. The author expresses heartfelt gratitude to the head 
of the Department of Zoology at the University of 
Tartu, Professor Kh. Kh. Riikoya, for the photo 
reprinted here of A. E. Grube. 


Shortly after graduation from the university, Grube 
left for a trip to the Mediterranean seacoast. "Baer's 
observations on the history of animal development," Grube 
wrote, "accompanied me on the trip to Italy, and along with 
the enjoyments of nature and art I continuously sated my 
interest in this sphere of science" Cp. 1). On the coast 
Grube zealously collected zoological materials and made 
dissections, considering these studies important for his 
future embryological investigations. His attention was 
attracted mainly to annelids, the study of which became 
his basic zoological specialty. Only in 1839 did Grube turn 
to work directly on annelid development, beginning with 
the study of the very small ova of the Saenuris vaviegata 
described by Hofmeister. In addition he reported that their 
development differs in many relations from the corresponding 
phenomena in medical leeches which had been very superficially 
described by E. Weber. The following spring, Grube studied 
the embryology of the leech and confirmed the reality of 
the formerly discovered differences. Preparing the results 
of his observations for publication, Grube conscientiously 
studied the works of the authors whom he considered his 
predecessors. The work of Filippi on the anatomy and 
embryology of Proboscidae leeches,! 5 recently published, 
especially interested him. Grube found that in Filippi' s 
investigations there was only scanty information about the 
division of the ovum, as the Italian author saw only "six 
lobes, situated in one and the same plane around the seventh, 
situated in the center." To this description he added that 
these spherical segments disintegrate into smaller spheres 
(GLOBULI ORGANICI), sharing in the formation of the 
embryo. Filippi apparently studied the developing ova through 
the membrane of a cocoon, which can explain the indistinct 
results of his observations. The first foundation of the 
embryo Filippi described with an indefinite expression-â€” cutis 
(LA CUTE) . He also erroneously considered, that the 
hatched young is nearly similar to the adult leech and at 
once is attached to the mother by the help of posterior 
suckers (in fact they are not present at this time) . 

Grube could trace the development of Clepsine in 
incomparably more detail; his success was aided by the 
application of concentrated reagents, in particular diluted 
nitric acid. In the first section of his work, Grube 
described the structure of the female and male sexual organs, copulation, the act of deposition of eggs, and also 
the structure of the ova at the time of maturation and 
directly after deposition. 



15. F. de Filippi, LETTERA AL S. DOTT . RUSCONI SOPRA 
L'ANATOMIA E LO SVILUPPO DELLE CLEPSINE (Pavia, 
1839) . 



The still-undeposited egg, hung in a special pocket 
in the oviduct, consists of a fine-grained substance 
(molecular bodies) and yolk (fatty bodies) ; it is provided 
with a nucleus (embryonic vesicle) . Before the deposition, 
the ovum, which is separated from the oviduct and is freely 
situated in the egg-reservoir, has the same structure; the 
nucleus already cannot be seen in a beam of light and can 
be seen only during the crushing of the ovum. Later the 
embryonic vesicle disappears completely. 

The second section of the work was devoted to the 
description of embryonic development. In the deposited ovum 
for one hour no change can be seen, and then at its poles 
the following phenomena can be observed. At the beginning, 
on one of them, a white spot appears; this increases and 
turns into a disk with a grey spot in the center (Figure 36, a). 
Then this grey center increases and a white spot appears in 
it, and the external white disk is transformed into a ring, 
which Grube called the polar ring (Figure 36, b) . The field 
of formation of the polar ring he called the active pole, 
since according to his observations, it is here at the time 
of development of the embryo that the most noticeable changes 
take place. On the opposite inactive pole a white ring appears, 
but less distinctly delimited. The polar ring ascends over 
the surface of the ovum in the form of a papilla, so that 
after its infiltration its condensation can be prepared. 
Grube considered that the formation of polar rings is the 
result of the displacement of an internal substance, during 
which the "molecular bodies" are gathered in the form of 
rings on the poles. 

Following the appearance of polar rings, or, in Grube* s 
expression, "the process of formation of fissures 
(DURCHFURCHUNG) or, rather, cleft (ZERKLUFTUNG) . . . . 
those fissures not only occur on the surface, but penetrate 
deep into the mass of the yolk, as a simple experiment shows: 
under careful pressure of the condensed yolk, it disintegrates 
into as many parts as the segments are delimited by the 
fissures" (p. 17). The first fissure divides the ovum into two unequal parts [Figure 36, c), so that the polar ring can 
be divided into halves, or from it a small part is separated, 
or the fissure occurs in direct proximity from the ring, 
not touching it completely. The second fissure divides the 
small segment approximately at a right angle to the first 
(Figure 36, d), and the third also divides the large segment 
(Figure 36, e] . 

Thus, the ovum passes into the stage of four blastomeres 
(Grube noted that the large segment in many cases is divided 
earlier than the smaller one) . Of the four segments formed, 
one is larger than all the others, and it is divided by a 
fourth fissure which provides the beginning of the fifth 
segment. The following division is concerned with this last 
and takes place not along its length, but transversely, the 
result of which is that the divided fifth segment forms a 
region in the form of a polar field in the inactive pole 
of the ovum (Figure 36, f) . The sixth, seventh and eighth 
fissures again go from the active pole to the polar field of 
the inactive pole. After the sixth division the polar ring 
on the inactive pole usually disappears. Grube did not see 
more than eight meridional divisions, and said that in 
leeches the blackberry stage characteristic for many animals 
is absent; in this stage the meridional divisions are replaced 
by transverse ones. 

At the same time as the division, accomplished by 
means of fissures, in the ovum of Clepsine the process of 
separation of the small globules from the large segments of 
the ovum first described by Grube takes place; these globules 
Grube called WANDUNGSBALLEN . In his opinion, the wall 
of the body of the embryo is built from them; "Segments of 
the yolk" and Wandungsballen are nothing more than the 
macromeres and micromeres of the terminology of today's 
embryology. The first small globule, as Grube noted, appears 
on the active pole after the formation of the first fissure 
and appears to be situated in this fissure. Judging by time 
and place of the appearance of this globule, here the 
discussion does not concern the formation of the micromeres, 
but the separation of the first polar body. The following 
small globules, forming on the active pole of the ovum, 
undoubtedly are micromeres. Grube described their accumulation, ascertaining during this that by the increase of the 
number of fissures the number of micromeres increases also; 
however, these phenomena do not stand in an indissoluble connection (Figure 36, e and g) . The formation of micromeres, 
according to Grube's description, is accompanied by the 
replacement of substances inside the ovum segments; he 
described these replacements in detail, alternating the 
statement of facts about his observations with theoretical 
conjecture about the forces of attraction, points of their 
application, and so on. 

The micromeres, accumulated on the active pole, do not 
exhibit equal size; the smaller they are, there are relatively 
larger molecular substances (protoplasm) anc i smaller fatty 
globules (yolk) in them. The smallest consist only of protoplasm and a round transparent nuclear body. These micromeres, 
according to Grube, never arise from macromeres; therefore, 
he wrote, "I must conclude that the small globules originate 
from those large globules already present on the pole which 
earlier separated from segments of yolk" (p. 22). The 
accumulation of micromeres forms a plate in the form of an 
isosceles triangle, without sharp limits passing into the 
other surface of the ovum. This plate, which Grube called 
the embryonic field, is so situated that the summit of the 
triangle is turned toward the active pole and corresponds 
to the cephalic part of the future embryo. 

It consists of "mosaic pieces" which arise as a result 
of the multiplication of micromeres; in addition, these 
pieces are smallest when they are nearest to the cephalic 
end. In the stage described, not only the anterior and 
posterior ends of the embryo, but also its upper and lower 
sides, as the surface of the embryo, on which the embryonic 
fluid is situated, correspond to its abdominal side. Using 
the descriptive phrase "mosaic pieces", Grube resolved 
consciously to avoid using the term cell, not being sure 
that it inherits its own features, which are characteristic 
of typical, in particular plant, cells. 

The anterior end of the embryonic field quickly expands 
(Figure 36, c) , and by forming two summits it produces S-shaped 
curved shafts (Figure 36, j), which Grube called the abdominal 
shafts. This name is well-founded, because these shafts, 
appearing in the field of the active hemisphere of the ovum, 
i.e. the future dorsal side of the embryo, are displaced 
towards its ventral side, where they later accrete into a single ventral embryonic region. The ventral shafts, 
according to Grube, on the ventral side of the leech embryo 
play the same role which the dorsal shafts play in the 
vertebrate embryos, because from them the wall of the body, 
in particular its muscles, is formed. The ventral shafts are 
composed of extremely small, closely adjacent other globules. 
At the posterior end of the embryonic field remain large 
globules which form part of its molecular (protoplasmic) 
content; three of these terminal globules usually are present 
on each side (Figure 36, k) . The internal part of the yolk 
globule (the divided ovum) constitutes a mass containing 
little protoplasm. This rich yolk mass is used mainly for 
building organs which are present in the abdominal cavity, 
in particular the intestinal canal; above it a layer grows, 
composed from a molecular mass which takes its origin from 
the ventral shafts. 



Figure 36. The development of Clepsine (by Grube) . 

a â€” fertilized ovum, bâ€” ovum on which the white disk with grey 
ring appeared, câ€” first fissure; the white disk is present 
near it. d â€” yolk globule, divided into four parts, eâ€” yolk 
globule, divided by six transverse fissures; the polar field 
increases as the surrounded globular segments decrease in size 
due to the loss of albumen (molecular) mass used for the formation of WANDUNGSBALLEN on the active pole, f â€” -the same stage, 
globules of division from the lower side; on the inactive pole 
the polar field is seen, i.e. the surface of the seventh segment, 
separated from the other round fissures, g â€” somewhat later 
stage, view from, above; the number of WANDUNGSBALLEN increasing, they then form a small disk which can be regarded as a 
rudiment (KEIM). h â€” WANDUNGSBALLEN increase in number and 
occupy a great area, forming the embryonic field, i â€” the view 
of the abdominal shaft from behind; they move so far apart that 
they envelop the yolk globule by a ring . jâ€” the abdominal shaft 
from the side; on the posterior end of each, three whitish 
globules are present, k â€” the abdominal shafts unite together. 



The rudiment of the neural cord appears in the form 
of two white stripes, joined to the external side by the 
recently formed embryonic stripes. After their closure on 
the ventral sides, the halves of the paired rudiment of the 
nervous system are united. 

In this period the surface of the embryonic body is 
already covered with epithelium, composed of flat cells of 
different sizes and forms. At the same time in the body 
cavity dissepiments are situated, their number gradually 
increasing. 

In the third part of Grube f s work he included changes 
occurring after the hatching of Clepsine from the egg 
membrane. Contrary to Filippi, Grube found very essential 
differences between the just-hatched leech and the formed 
worms. In the short, cylindrical little worm hatching from 
the egg, there are still no posterior sucking discs, no eyes, 
no blood vessels, and the formation of the dissepiments is 
also unfinished. As the young larvae are also immediately 
fixed by the anterior end, Grube assumed the presence of the 
rudiment of the anterior sucker with longitudinal and circular 
muscles. Within one day after hatching this sucker is clearly 
noticeable in the form of a convex ring; after two days 
this ring elongates, as Grube thought, under the effect of the 
heaviness of the body of the leech hanging on it. Inside the 
elongated ring a canal appears representing, according to Grube, the rudiment of the sheath of the still unformed 
proboscis. The wall of the intestinal canal, or yolk sac, 
consists at this time of large cells. Later, by means of a 
circular twist of the most anterior part of the body, the 
caudal disc develops and the proboscis is formed, which the 
larva can let out and pull in. The digestive canal is formed 
in this way. On the intestinal canal extensions appear, and 
in addition to this, it is divided into three parts. The 
anterior and posterior parts are significantly thinner than 
the middle one which composes the future stomach. In the last 
the more enlarged lateral pouches appear. Later on the eyes 
develop; at first they have the form of circular red spots. 
The vascular system develops, and, on the dorsal side of the 
body, the pulsating heart first becomes noticeable. 


Grube finished the description of his observations with 
the following words: "The development considered here is 
probably spread extensively in the class ANNULATA. This 
is established on the basis of detailed investigations on many 
representatives of Naidae and Lumbricinae namely in Saenuris 
variegata Hoffm., Euaxes acutirostris Gr. and Lwnbvioulus 
variegatus Gr., and if it can be judged by an analogy with 
the adult animals, it will be also correct for the genus 
Lumbricus, and also for many sea worms" Cp. 45). With this, 
Grube carefully observed that discussion of analogy without 
special investigations can lead to erroneous conclusions. 

A discussion of Grube' s work leads to the conclusion 
that he promoted the study of embryology of the annulated 
worms (annelids) and discovered phenomena the detailed study 
of which was done significantly later. His most important 
achievement in terms of recent embryology can be summarized 
in the following way: 

1. Grube established that in the eggs of Proboscidea 
leeches there are polar plasmas; especially conspicuous are 
protoplasmic rings on the animal pole. 

2. He recognized the complete, unequal division of the 
ovum of Proboscidea leeches, during which the animal polar 
plasma can go in one of the blastomeres of the four-celled 
stage. 

3. Grube clearly saw how in the stage of the four 
blastomeres, from the animal side, very small globules of 
division began to separate; i.e. he established the fact 
of the formation of micromeres, which are composed mainly 
of protoplasm, and macromeres, which are rich in yolk. 

4. Later he established that the number of micromeres 
increased either by separation from macromeres, or by 
means of multiplication of the early formed micromeres. 

5. By Grube' s observations, the "rudiment" which is 
formed on the animal pole, owing to the multiplication of 
micromeres, spreads over the surface of the ovum, as a 
result of which the macromeres appear inside the embryo. 
This phenomenon very nearly resembles epibolic gastrulation. 

6. At the end, Grube described the embryonic stripes 
coming out from three pairs of terminal cells; i.e. he 
discovered in the annulated worms the phenomenon of 
teloblastic development. 

7. The embryonic stripes, in Grube' s observations, 
are displaced towards each other and are united on the 
ventral side. From the material of the embryonic stripes 
("the abdominal shafts") the wall of the worm's bodytegmens, muscles, and nervous system is formed. 

Grube 's excellent work was forgotten, and superiority 
in the study of the embryology of annulated worms, in 
particular the leeches, was accorded to Rathke, whose work 
was published eighteen years later. Rathke 's work without 
doubt has merit, but also has many defects, so that it 
can be seen as a step backwards in comparison with the work 
of Grube. Thus in the maxillary leech Nephelis vulgaris 
Rathke clearly saw the formation of micromeres, which in 
Clepsine complanata he did not see, and instead of the 
micromeres only granularity is illustrated in his drawing 
in the animal parts of the blastomeres. At a later stage it 
is shown as if the unlacing of the micromeres has begun, but 
he did not see whether this led them to division. Together 
with this, Rathke reproached Grube for seeking the source 
of micromeres CWANDUNGSBALLEN) in the depths of the 
globules of the division, while Rathke himself in general did not see their separation. According to his description, 
a number of hillocks on the animal pole of the divided ovum 
directly turned into a thickening which is the rudiment of 
the embryonic stripes. The embryonic stripes of Nephelzs 
are illustrated very schematically by Rathke; he did not 
observe the characteristic cellular rows in these stripes, 
neither did he recognize the connection between the only 
three teloblasts illustrated in his drawing which are 
situated as if they lie at the extreme ends opposite to 
the embryonic stripes and the middle one as if it lies 
between these ends. Rathke also represented the embryonic 
stripes of Clepsine less distinctly than Grube. 

All this is mentioned not to underestimate the 
significance of Rathke ! s investigations, but only to drawattention to the more perfect work, in many aspects, of 
Grube, to call to mind its priority and to show the importance 
of his work in the history of Russian and world embryology. 

In the 1830' s and 1840' s, the development of invertebrates also interested A. D. Nordmann (1803 - 1866). Aleksandr 
Davidovich Nordmann was the son of a Russian officer born 
in Finland. In 1821 he entered the university in Abo, and 
after graduation he worked for some years in Berlin with 
Rudolphi and Ehrenberg. During his stay in Germany he 
accompanied Oken, Tiedemann, and Chamisso on a trip for 
the study of sea fauna. In Berlin Nordmann published his 
first work, MICROGRAPHICAL INFORMATION, dedicated to 
the structure and taxonomy of parasitic worms (describing 
many new forms, in particular "spainika" - Diplozoon paradoxwi) , 
and also the structure and development of parasitic copepods.16 
In 1832 Nordmann was invited to join the department of zoology and botany of Lyceum Rishel in Odessa, and in the following 
year he took the post of director of Odessa Botanic Garden 
C128) . In 1833 Nordmann together with Rathke, S. S. Kutorgaya 
and Steven travelled to the Crimea. In the following years 
he travelled much in the south of Russia; in particular, he 
led excursions in the Crimea of students of the Odessa 
Lyceum. When he was sent with a scientific mission to Paris, 
Nordmann visited with Milne-Edwards the coast of Normandy. In 
1849 Nordmann came to Helsingfors University, where from 
1852 to the end of his life he headed the department of 
zoology. During the period of his scientific activity, 
Nordmann published fifty-seven works in Russian, Latin, 
German, French and Swedish on anatomy, embryology, taxonomy 
and zoography of different groups of vertebrates (mammals, 
birds, fish) and invertebrates (insects, spiders, crustaceans 
and worms, mainly the parasitic molluscs, bryozoans and 
Coelenteratae) ; he also studied botany and paleontology. 




16. A. v. Nordmann, MIKROGRAPHISCHE BEITRAGE ZUR NATURGESCHICHTE DER WIRBELLOSEN THIERE (Berlin, 1832), 
Vol. I, viii + 118 pp., 1: UBER BINNENWURMER IM AUGE 
HOHERER THIERE; 2: BESCHREIBUNG EINIGER NEUEN 
HELMINTHE. vol. II, xviii + 150 pp., 1: BESCHREIBUNG 
EINIGER NEUEN PARASITICHEN ENTOMOSTRACEEN; 
2: ERSTER BEITRAG ZUR NATURGESCHICHTE DER 
LERNAEN. 



In the previously mentioned MICROGRAPHICAL INFORMATION, the results of Nordmann ! s observations on the 
development of parasitic copepoda Aahtheres peroanon appear, 
and the larval stages of other related forms (Ergasilus 
Sieboldi Nordm. , Traoheliastes polycolpus Nordm. and 
Lernaeoaera cyprinacea) are described. These investigations 
met the need to explain the systematic situation of parasites, 
which at that time were related either to molluscs or to 
the annulated worms, or even to coelenterates. If the relationship of some representatives of the mentioned groups CfÂ° r 
example, Caligus) to crustaceans was to some extent only 
probable, then these forms, such as Lemaeocera , which in 
the adult condition are completely unlike arthropods, 
remained in their systematic relations mysterious. 

The embryonic development of Aahtheres was described 
by Nordmann rather incompletely. "On the upper surface of 
the yolk," he wrote, "is found at first a more transparent 
region, an4 a granular part of the yolk, having the significance of a rudiment (KEIM) , turned into round or spherical 
forms from which the more peripheral give material for the 
formation of the rudiment membrane (KEIMHAUT) ... The 
latter completely envelops the yolk and... forms later on 
the wall of the body of the embryo (p. 78). After referring 
to the separation of the head and the appearance of the rudiments of the extremities, Nordmann turned to the 
characteristics of the nauplius larva, whose structure 
becomes complicated after moulting. 

Its central point was the description of the larval 
stage of Lemaeooera oyprinaoea . The adult animals of 
this genus were already known to Linnaeus; they are 
characterized by a sacculated unopened body, deprived of 
extremities and organs of sensations (Figure 37, A). "If 
naturalists," Nordmann wrote, "are astonished at the 
structure of the body of the mature animal of this kind, 
their astonishment will be more natural when an opportunity 
arises for them to observe the young animals. It can hardly 
be imagined that there is anything more striking than an 
offspring having absolutely nothing in common with its 
parent. Before my eyes the egg receptacle in the mature 
animal was ruptured, and the embryos set free. I saw young 
animals exactly the same as I represent them in the drawing 
(Figure 37, B) ; they have extremities, antennae, and even 
bright red eyes" (128). 

Nordmann 1 s observations met, according to him, decided 
distrust from Berlin zoologists to whom he demonstrated 
the nauplius larvae of Lemaeooera oyprinaoea. Later the 
significance of this discovery was universally recognized 
(129) . 

After some years Nordmann published a small embryological 
work concerning the Black Sea bryozoan Tenchca zosterioola A 7 
In one zooid Nordmann found from four to seven eggs and 
saw the penetration of the spermatozoids in the female cells 
through an opening in their base. Later he saw the embryos 
hatched from the eggs swimming by means of cilia and finally 
settling in the seaweed Zostera. "As far as it is possible," 
Nordmann wrote, "I observed the transformation of the young 
animals and the development of polyps from them." Greater 
significance was possessed, however, by other embryological 
work of Nordmann' s on the development of molluscs. 



17. A. v. Nordmann, "Recherches microscopiques sur l'anatomie 
et le developpement du Tendra zosterioola, espece de 
polype de la section des Bryozoaires , " ANN. SC . NAT. 
ZOOL., 11 (1838), pp. 185 - 191. 





Figure 37. Lernaeooera cyprinacea (A) and its larva (B) (by Nordmann) . 


The molluscs, especially the gastropods and lamellibranchs, are considered easily available material for the 
study of development processes; therefore they early 
attracted the attention of embryologists . The division of 
the ova of the gastropod molluscs is described in the 
investigations of Van Beneden and Windischmann, and the 
division of the ova of the lamellibranchs â€” in the work of 
Loven. In the ova of Modiolaria marmorata {Mytilus aisoors) 
Loven observed a maturation division and described it, as 
a process of ejection of nucleolus (embryonic spot) . The 
important result of his work was the establishment of the 
fact that during the division of the ovum of Modiolaria, the 
separation in the vegetative hemisphere of non-nucleated 
lobes takes place which soon merges with one of the blastomeres. This phenomenon is repeated many times during the 
time of the following divisions. Loven saw clearly the 
process of division itself, i.e. the deviation of the ovum 
into separate blastomeres, but he did not trace the changes 
in the nuclei, the equal and mutual distribution of blastomeres . 

The separation of polar bodies in the ovum of the grey 
slug is sufficiently clearly described in the above-mentioned 
works of Van Beneden and Windischmann, although the division 
following the separation of the polar bodies was inaccurately 
described by them; they spoke about the formation of elevations on the surface of the yolk, divided by fissures. As 
a result of this the entire surface of the ovum becomes at 
the end as if embossed (BOSSELE) and resembles a raspberry, 
The Belgian authors did not connect the formation of "the 
yolk cells" with the appearance of the cuts on the surface 
of the yolk; according to their concept, the yolk cells 
originate under the superficial layer of the yolk. The 
drawings illustrating their work show round bulges on the 
surface of the developing ovum, and not its division as a 
whole into separate blastomeres. The internal processes in 
the divided ovum and blastomeres are neither described nor 
illustrated in the drawings. Erroneous ideas of the processes 
of division in the gastropodan molluscs were retained in the 
work of 0. Schmidt appearing ten years later. 18 "The description of the ovum and the processes of division 
inside it up to the formation of the embryo," Schmidt 
wrote, "was given so completely by van Beneden and 
Windischmann that I cannot add anything to it" (p. 278). 




18. O. Schmidt, "Uber Entwickelung von Limax agvestis , " ARCH ANAT., PHYSIOL. (1851), pp. 278 - 290 . 




The subject of Nordmann's above mentioned work on the 
development of the gastropodan molluscs was the nudibranchiate 
mollusc Tergipes Edwardsii . The description of the phenomena 
of its embryonic development constitutes part of the extensive 
monograph, 19 beginning with the presentation of anatomical 
data. Nordmann described in detail the development of the 
ovum in the ovary (5 28 - 31), characterized the yolk 
elements of the ovum (Â§ 32) , the structure of the egg 
ready for oviposition (5 33) and its membranes (5 34) . The 
process of division {Â§ 39) of the ovum of Tergipes was 
described by Nordmann as the following: the first fissure 
of division can take place in different directions and 
divide the ovum either into equal, or into very unequal 
parts. The fissure dividing the ovum into four globules 
of division takes place at right angles to the first 
fissure, after which the ovum is divided into eight globules, 
and so on. The generally uneven character of division is 
noted in the following expression. "Although the tendency 
to even progressive division cannot be disputed, it also 
shows that the yolk, especially up to its acquisition of 
the mulberry form, appears to divide very unevenly" (p. 573) 
(Figure 38, a-g) . Nordmann noted especially that the fissures 
cut all the mass of the yolk, i.e. that the division was 
complete. 




19. Nordmann, "Versuch einer Natur- und Entwickelungsgeschichte 
des Tergipes E 'dwards ii , " MEM . , pres . a l'Acad. Sc. 
St.-Petersb. par divers savants, 4 (1845) , pp. 495 - 602. 
A brief translation of this work in French was published 
by K. Fogt ("Essai d'une monographie du Tergipes Edwardsii," 
ANN. SC. NAT., 3, ser. Zool., 5 (1846), pp. 109 - 160). 


All the course of development of Tergipes Nordmann summarized in the following statements: 


1 . The chorion extend to one-fifth the diameter of 
the ovum and becomes oval . 2 . At this time a light 
fluid, similar to albumin, discharges from the ovum. 
3. The yolk loses its spherical form, its mass 
loosens, and the contours become wrinkled. 4. The 
embryonic vesicle and the embryonic spot disappear. 
5. The upper layers of the yolk lose their reddish 
coloration. 6. In too many cases separate yolk 
cells are separated from the other mass of the 
yolk and give origin to the parasitic creatures. 20 

7 . The ovum is divided by a fissure into two globules . 

8. The division process continues regularly. 

9. The yolk acquires the form of a mulberry. 

10. A bubble of air leaves the yolk (?) . 11. The 
surface of the yolk becomes granular. 12. First 
establishment of the embryo. The yolk acquires an 
elongated form, and then the form of a roughly 
outlined triangle. 13. The distinct appearance of 
the animal system and the cutaneous system. The 
configuration of the embryo. 14. Twisting of the 
anterior part of the embryo is noted (future 
organs of movement) . 15. On the wide anterior area 
folds appear, from which two lateral round growths 
are gradually formed. The growths are transformed 
into lobes and between them a third growth, the 
foot, appears. 17. Beginning of the formation of 
mantle and concha. 18. On the lobes the cilia grow. 
19. The first movement of the embryo. On the foot 
the vibrating cilia appear. 21. The lobes of the 
sail become disc-shaped. The rotatory movement of 
the embryo. 22. Cells sharing in the formation of 
mantle are dissolved and disappear. 23. The concha 
significantly enlarges. 24. The isolated cellular 
rows indicate the formation of the attached muscles . 

25. The formation of the internal organs, among 
which the intestine is distinctly differentiated. 

26 . The liver and other glandular bodies are clearly 
seen. The anus and ganglia. 

27. The cells forming the attaching muscles disappear. 28. Deposition of 
the pigment in the eyes . 29 . Between the chorion and 
the embryo the astonishing parasites appear to rush 
out. 30. The complete formation of the embryo larva; 
the roof of the shell opens and closes. 31. The 
extension of the chorion. 32. The presence of the 
young in the common ovum chamber. 33. The larva cuts 
the chorion. 34. Hatching, (pp. 565 - 567) 




20. Nordmann gave this parasite the name Cosmella hydrachnoides 
and suggested the possibility of its origin from particles 
of the yolk of Terg-ipes. 








Figure 38. The development of Tergipes Edwardsii (by 

Nordmann) a-g â€” division of the ovum; h â€” larva, 



,36 



From the above it can be seen that Nordmann's main 
attention was to the relatively late stages of embryonic 
development, while he investigated the process of division 
of Tergipes only extremely incompletely. 



The brief discussion of the works of van Beneden, 
Loven and Nordmann on the embryology of molluscs demonstrated 
the level of knowledge in this sphere at that time. When 
the development of molluscs was studied by the Russian 
embryologist N. A. Warnek, his work can be unconditionally 
called classical (131). Warnek's scientific activity, unfortunately, was brief and was cut short by external circumstances, 

Nikolai Aleksandrovich Warnek was born in 1821. When he 
was eighteen years old he entered the faculty of law at 
Petersburg University, but in the same year he transferred 
to a department of the faculty of philosophy. During his years 
of study, Warnek received in 1843 the gold medal for his 
"Process of Moulting of External Tegmens and Formation of 
Millstone in the Ordinary River Crayfish." In 1846 he graduated 
from the university with a candidate's degree, and for three 
years he taught botany and zoology in Gorn Institute. Warnek 
received his master of science degree for his work on the 
structure and function of the crayfish liver^l and in 1849 
he started reading lectures to naturalists and medical men 
in comparative anatomy and physiology in Moscow University, 
first as junior scientific assistant and then as extraordinary 
professor (from 1852) . In 1848 Warnek wrote the vast work 
"On the Formation and the Development of the Embryo in the 
Gastropodan Molluscs," which was published two years later 
in the BULLETIN OF THE MOSCOW SOCIETY OF NATURALISTS; 22 a summary of this work was published abroad under 



21. N. A. Warnek, "On the Liver of River Crayfish in 
Anatomical and Physiological Relations" (SPb., 1847), 
40 pp. 

22. warnek, "Ober die Bildung und Entwickelung des Embryos 
bei Gasteropoden," BULL. SOC . NATUR . MOSCOW, 23 

(1850) , pp. 90 - 194. 



the title "On the Process of Division and Structure of the 
Ovum in Gastropods. "23 

In 1860 Warnek retired and departed to Tver; where for 
three years he was director of a secondary school, and then 
director of the schools of Tver Province. 

Until recently it was thought that Warnek 's scientific 
and literary activity ended in 1863 and that in 1867 he died. 
From recent work by T. P. Platovaya 2 ^ it is now known that in 
1880 and 1881 Warnek read a report to the Moscow Society of 
Naturalists on the biology of agricultural pests, and in 
1884 a report on the microscopic structure of the fish ovum 
and on the morphology and taxonomy of fish. The exact date 
of his death remains unknown. 

Concerning his retirement from Moscow University evidence 
has been kept which is probably not completely objective and 
in any case explains only incompletely the cause of his 
retirement from teaching and from scientific work. 

In the multi-volume apologetical biography of the 
reactionary historian Pogodin, its author N. Barsukov25 
reminisced about I. A. Mitropolsky, who was in 1850 a student 
in the Faculty of Medicine in Moscow University. According 
to Mitropolsky, when Warnek was reading zoology and comparative 
anatomy he was noted for an extremely scornful nature; due to 
this he was disliked by the students and medical men. One 
of Warnek 's clashes with the students, Mitropolsky reported, 
ended in a noisy scandal, in which both students and medical 
men participated. In order to confirm to the reader the 
ultimate rightness of the students, Mitropolsky declared that 
"the students could endure the tricks of the professor only 
if his lectures gave them anything useful. But they got 
nothing from them." 



23. Warnek, "Uber den Furchungsprozess und die Struktur des 
Eies der Gasteropoden," FRORIEP ' S TAGESBERICHT , 
No. 280 (1851) , pp. 43 - 44. 

24. See (131) . 

25. N. Barsukov, ZHIZN'I TRUDY M. P. POGODINA (Life 
and Works of M. P. Pogodin), Vol. 16 (1902), pp. 116 - 117 


Another aspect of this incident is mentioned in the 
reminiscences of L. V. Lebedinsky, which were published 
in VOICE OF THE PAST in 1912.26 Lebedinsky characterized Warnek as talented, but very proud and sharp in his 
treatment of people, who "excited against himself the 
students and medical men by his tactless relations with 
them and an unusual strictness during examinations." 
Handbooks in Russian in zoology and comparative anatomy 
were absent at that time, and Warnek suggested foreign 
books, which the students, due to their ignorance of the 
languages, found difficult to use. He did not read a 
systematic course, but selected works of his own, which 
he considered more important and interesting. In answer to 
the protests of students on this occasion Warnek declared 
that "the students of a university are not secondary schoolboys, they must work independently. The professor in his 
lectures has only to point out a direction and a method by 
which the students must carry out their work." The dissatisfaction of the students and medical men was expressed 
in the organization of meetings in which the students of 
other faculties shared, students of law and philologists 
who did not attend the lectures of Warnek and could not 
judge their effects and defects. In these meetings it was 
decided to criticize the professor, and then oblige him 
to leave the department. Only some of the students and 
medical men, Lebedinsky continued, strongly defended the 
professor. "They said that Warnek was regarded differently 
and they loved him for his talented presentation of the 
subject." The planned obstruction was nonetheless carried 
out, mainly by students of other faculties who shared in 
the action, especially those of the faculty of law. This 
produced on Warnek a stunning effect. "Warnek went into an 
adjacent room," Lebedinsky wrote, "where he fainted away; it is even said that blood gushed from his throat." Soon 
after this incident Warnek retired, insisting on it despite 
intensive persuasion. 



26. UZ ZHIZNI MOSKOVSKOGO UNIVERSITETA. VARNEKOVSKAYA ISTORIYA. "GOLOS MINUVSHEGO" (From the life 
of Moscow University. Warnek' s History. "Voice of the 
Past") (Otd. ottisk, pp. 210 - 218). For this source, as 
well as for the answer in HERZENOVSKY "K0L0K0LE" (see 
[132]), the author expresses deep gratitude to V. V. 
Sorokin . 



Already this comparison of evidence shows the impossibility of placing complete responsibility on Warnek himself 
for what occurred. The objective evidence of Lebedinsky about 
Warnek f s talented lectures to the students and medical men 
and their good relation to him unmasks the tendentious 
assertion of Mitropolsky that Warnek' s lectures gave nothing 
to the students. The impression one has of Warnek is of a 
serious, exacting professor who loved his subject and taught 
it on a high level. Certain students and medical men understood his efforts and valued his services to them, while 
the majority of students were interested in the applied 
questions and not in understanding Warnek' s theoretical 
views, and saw him only as a strict examiner who shamed them, 
in addition to his ridiculing of their ignorance. 

In all this sad "history of Warnek" there is still one 
significant side which can be read between the following 
lines in Lebedinsky 's memoirs: "Even among the professors 
there were people who sympathized with the students, the 
professor of theology Sergievsky among them. This handsome, 
somewhat fanciful, young orator was sometimes present in the 
department. He unnoticeably and cleverly approached the evils 
of the day, said a few words hinting at an excellent understanding of the students, and was zealously rewarded with 
applause. In the time of the aforementioned event, one of 
his lectures, which was full of these hints, ended with the 
following significant words: 'Yes, this darkness does not 
triumph over the world!"' 

It is logical to ask why Warnek was not pleasing to 
the professor of theology. What world of darkness and damage 
spreading from Warnek 's department was frightening to 
Sergievsky? To answer this question is not difficult. Warnek 
was a convinced materialist, as is easy to ascertain from 
a study of his basic work. It is improbable that in his 
lectures Warnek did not touch upon questions of ideology, 
which, of course, was used against him by Sergievsky, who 
considered it his responsibility to protect the world of 
"religion" from the "darkness" of the natural-scientific materialism. Sergievsky's Jesuitical activity made the best 
use of the dissatisfaction of the protesting group of 
students, who played a role obviously of considerable 
importance in the organization of the obstruction, which 
took place with the obvious support of the administration 
of the university, usually so vigilant when the matter is 
concerned with student disturbances (132) . 

The fault, or rather misfortune, of Warnek was his 
sharp and derisive character, his inability to adapt 
himself to his surroundings. Warnek did not want to 
reconcile himself with the manifestations of ignorance, 
even if its carriers were respectable scientists. This 
can be witnessed by Warnek 's review of a scholarly book 
by A. Bogdanov. 27 This review shows great and extensive 
knowledge and unquestionable educational talent of the 
reviewer and his understanding of the problem of the teaching 
of natural sciences. In addition, the review is written in 
an overly particular and caustic tone; Warnek did not let 
pass any mistake, any slip of the tongue, or any lame expression from the author of the book. It is easy to imagine what 
reaction Warnek' s article could produce in Bogdanov. If 
Warnek behaved similarly in the professional milieu of 
Moscow University, and used to deride the lectures of his 
colleagues, then he undoubtedly provoked against himself 
not only a certain group of students, but also some professors. 

Valuable evidence of Warnek 's high character as a lecturer 
and scientific worker is contained in the words of I. M. 
Sechenov : 28 

Junior Scientific Assistant Warnek taught us zoology. 
He read simply and clearly, dwelling mainly on general 
signs applied in zoology departments, and the description of protozoa was prefaced by a long treatise on 
the cell in general. This last study was built, however, 
on unprepared ground. Moscow still did not think much 
at that time of the microscope; among Warnek' s students 
it was not used successfully, and in mockery they 
nicknamed him "Cellular." 



27. Warnek, "Zoologiya i zoologicheskaya khrestomatiya v obieme 
srednykh uchebnykh zavedenii , " Author Anatolii Bogdanov, 
OTD. PERVY ZH. MNP, ch. 118 (1863), pp. 47 - 73. 

28. I. M. Sechenov, AVTOBIOGRAFICHESKIE ZAPISKI (Izd. AN 
SSSR, 1945) , 176 pp. 


In a footnote Sechenov added: "Too late, I have learned 
that Warnek and the famous botanist Tsenkovsky were among 
the first Russian biologists who worked at that time with 
the microscope." 

On Warnek' s socio-political opinions there is no 
information. His statement against Eogdanov's book, based 
on the principles of Darwin's evolutionary study, was 
published in a government journal; objectively, it could 
have played a reactionary role. The mutual relations of 
Warnek with the students creates the suspicion of the 
possibility that the rising conflict was of a political 
nature. Concerning Warnek 's "political orientation," it 
can be seen that he was later assigned responsible posts 
in the Department of Education. A search of the archival 
material may throw light on this subject. 

For a characterization of Warnek' s scientific work and 
world view, it is necessary to gain acquaintance with the 
contents of his basic work, "On the Formation and Development 
of the F:mbryo in the Gastropodan Molluscs. "^ As an epigraph 
to this work Warnek selected the words of Reii: "The phenomena 
of individual life are the necessary result of formation and 
merging." This idea, as can be seen from the final chapter 
of this work, Warnek interpreted materialistically. Warnek 
himself also considered that only in the definite mixing of 
substances in definite spatial position [form) can the 
solution of vital phenomena be sought. Science, in his 
opinion, is not in need of additional idealistic assumptions. 

As it is seen from the introduction of his work, Warnek 
considered his principal task to be the explanation of how 
the yolk, i.e. ^substances of the ovum, are transformed into 
tissue of the embryo and what conditions this transformation. 
The solution to this question, in Warnek' s opinion, is possible 
only through a thorough investigation of the processes of 
development, and that is why he also outlined the following 
vast program. First of all, Warnek suggested, it is necessary 
to study the reproduction of molluscs, either hermaphrodites 
or separate sexes. For this aim the following must be studied: 
1) structure of male and female sexual organs; 2) origin of 



29. See footnote 22 of this chapter 



embryo, i.e. development of yolk (ovum) and semen; 
3) processes occurring during copulation, i.e. the influence 
of sperm upon the yolk, and finally 4) formation of the 
additional parts of the ovum â€” albumin, membranes and ovum 
cocoons . 

Only after this it is possible to begin the study of 
embryonic development, which Warnek divided into two periods. 
The first period includes the development of the fertilized 
ovum, i.e. the process of division and preparation of the 
development of organs, and the second, the period of development of all systems of organs of the developing animal. 

In his investigations Warnek proceeded from the following 
situations established by the embryologists: 1) for the 
transformation of yolk into an embryo fertilization is 
absolutely necessary; 2) this last consists of the material 
influence of semen on the yolk; 3) this influence takes place 
only in an infinitely small space, therefore the spermatozoa 
of the semen must come in direct contact with the mass of 
yolk; 4) the result of fertilization is the formation of the 
elementary organs of the embryo (cells); 5] the cells acquire 
different forms, grouped in complicated organs of the embryo, 
and thus form its body. 

Warnek expressed regret that embryologists could not 
completely solve the following important questions related 
to the development of animals: 1) how the yolk, i.e. substance 
of the ovum is transformed into tissue of the embryo; 2) how 
its transformation at the time of development occurs; 3) in 
what does the secret influence of the semen on the yolk 
consist. 

The first question Warnek considered to be solvable, 
and he expressed the hope that in a short while the other 
two questions would also be explained. 

The solution of the principal questions of embryology, 
in Warnek' s opinion, could take place on the basis of the 
following presentations: 

The cause of the phenomena conditioning the beginning 
and the subsequent development of the embryo is 
ordinarily attributed to the vital power, which absolutely clearly shows that the beginning of life 
must be sought in the formation of the ovum in the 
ovary, and the beginning of its development in 
fertilization, namely in the influence of semen on 
the yolk. And, of course, the essence of the powers 
which condition all phenomena of nature remains 
unknown. If all this can reduce the different 
phenomena to one cause, this will make a great step 
toward the goal we have established, (pp. 94 - 95) 

During the last four years, aspiration to a solution 
of the aforementioned general questions was my 
dearest hope. Devoting myself year after year to 
the study of the formation, development, and 
functions of cells, I have stuck to the idea that 
elucidation of the ideas of development and activity 
of the cells is the only way to select from the 
labyrinth of recent presentations about the organic 
world. The explanation of the causes of the vital 
activity in the cells leads to a clear presentation 
about life in general and about causes directing it 
throughout the organic world, (p. 95) 

The following work of Warnek represents the first part 
of investigations of his planned program; it is concerned 
with ovum structure and processes of its division in the 
gastropodan molluscs. 

Warnek began with the description of the form of oviposition of the different fresh-water snails; he detailed the 
periods of oviposition in the region of Petersburg and the 
structure of gelatinous mass surrounding the ovum. Later he 
described the structure of the laid ova, especially in the 
species used in this work: Lymnaeus stagnalis and the slug 
Limax agrestis . Transferring to the principal part of the 
work, throwing light on the study of division, Warnek paused 
at the characteristics of the yolk granules filling the 
fertilized ovum. Among these granules Warnek saw a light 
spot which was not delimited from the yolk membrane and was 
always situated in the center of the ovum. All the process 
of division of the ova of the molluscs which he studied Warnek 
divided into stages, described in succession. 



FIRST STAGE: For this stage, characteristic phenomena 
are taking place in the above mentioned light spot, which, 
in Warnek's words, occupies the place of the rudimentary 
(embryonic) vesicle. This spot is in the beginning completely 
round (Figure 39, 3), then becomes elongated and subsequently 
takes the form of a biscuit and the shape of a figure eight 
(Figure 39, 4 and 5). After that when the spot (i.e. nucleus) 
is twisted by the means just described, it comes nearer to a 
certain region of the periphery of the yolk (ovum) . The end 
of the spot which is turned to the surface of the yolk widens, 
and it acquires the form of a blunt rounded cone, in addition 
between the outer area of this cone and the membrane of the 
yolk a transparent crescent-shaped region appears (Figure 39, 
7) . From this crescent-shaped region two small vesicles become 
separated, which, being isolated from the ovum, remain near it 
throughout the following development. The place of deviation 
of these vesicles becomes the center of formation of fissures, 
later dividing the yolk into two, then into four parts. 

The separation of the vesicles Warnek described as 
follows: on the external surface of the crescent-shaped region 
a small elevation appears under the yolk membrane. It 
gradually enlarges, acquiring the form of a spherical segment, 
a hemisphere, then a complete globule, which is set on a 
sufficiently thick stalk. Then this stalk becomes unlaced, 
and the globule becomes free (Figure 39, 9). After the 
formation of one vesicle the second one appears exactly as 
the first (Figure 39, 9 and 10). Thus, in Warnek's observations, the crescent-shaped region separates the forming 
vesicles from the light spot (i.e. from both nuclei of the 
ovum) . He concluded that the nucleus does not share in the 
formation, at least, of the external vesicles. He made this 
erroneous conclusion because the intravital observations which 
Warnek used did not suggest tracing the processes taking 
place in the nucleus. From this came the further erroneous 
claim that the separating vesicles could not be regarded as 
the vesicles of Purkinje or its remnants. In accordance with 
this assertion Warnek refused to recognize for the vesicles 
separated from the ovum that important role claimed for them 
by many authors, and he objected to the name "directing 
vesicle" {vesioula directrix') . Warnek did not like this 
name as it returned embryologists to the time when they 
believed in an Archean spirit directing vital phenomena. 


The presence of vesicles where the formation of fissures 
of division begins did not prove, in Warnek's opinion, that 
the topographical position of the fissures was determined 
by the vesicles. Preferably, as he thought, the matter was 
the contrary: the vesicles are separated where the center 
of division is present. In later stages of division many 
fissures appear without preliminary separation of the vesicles 

After the separation of both vesicles the transparent 
crescent-shaped region also disappears. In the ova of the 
slug there appear two nuclei distinctly separated from each 
other. At this time the nuclei each acquire distinct contour 
and a large nucleolus. 



THE SECOND STAGE begins with the loss of the membranes 
of the nuclei and their merging into one common mass. This 
mass acquires an extended form, situated on the longitudinal 
diameter of the yolk, i.e. at right angle to the position 
which is characteristic for the first stage (Figure 40, 11) . 
Then the nucleus becomes biscuit-shaped (Figure 40, 12) and 
at the same time the division of the yolk begins. In the 
last description Warnek used topographical terms, and their 
significance is explained in the literal remark. The transverse 
diameter he called the diameter; passing through the vesicles 
from the yolk at right angle to it the longitudinal diameter 
is situated. The terminal points of these diameters he called 
poles: the dorsal pole in the place where the external vesicles 
were present; the ventral pole situated against the dorsal one; 
the poles of the longitudinal diameter designated as right and 
left. 

The division of the yolk is preceded by a thickening of 
the dorsal pole, in its field; then a fissure in the form of 
a cut appears. Due to its deepening the ovum becomes in form 
more like the kidney (Figure 40, 13) . The direction of the 
fissure does not coincide with the transverse diameter; it 
is inclined to it at a 45-degree angle. The light spot 
decreases and becomes less noticeable even in the ova of the 
slug, and in (Lymnaeus stagnali-s) it is not seen in the 
majority of cases from the very beginning of the division. 



Figure 39 . Warnek ' s drawings for his work on the development of 
gastropodan molluscs (development of Lymnaeus 
stagnalis) 



3-5 â€” the division of the light spot in the fertilized 
ovum; 

7 â€” at the external area, light spot coming near the 
surface; a light crescent-shaped formation appears; the 
beginning of the formation of elevation; 

the elevation acquires the form of a club-head; 

9 the second elevation begins to form; 

10 â€” the second vesicle separates; the light spot becomes 
spherical; the crescent-shaped region disappears; 

19 â€” completely separated yolk globules; 

22â€” the beginning of secondary nearness of the yolk 
globules ; between them a light space ; 

25â€” the greatest nearness of the yolk globules; the 
nuclei are not seen; 

25aâ€” no n- simultaneous division of nuclei into two primary 
yolk globules; 

27â€” non-simultaneous division of yolk globules; 

28-30 â€” gradual distortion of first fissure of division; 

3 4 cross -shaped position of the globules of division; 

36a â€” no n- simultaneous division of the nucleus into four 
yolk globules; 

40â€” globules of division of the fourth stage (la- Id) are 
situated in the space between globules of the third stage 
CIA- ID) ; 

43â€” extension of nuclei in the yolk globules of the 
third stage (1A-1D) ; 

45â€” the yolk globules of the fifth stage (2a-2d) 
separated from the globules of the third stage (2A-2D) 
and situated between the latter; 

46a â€” adjacent situation of nuclei, showing the origin 
of the yolk globules of the fifth stage from the globules of 
the third stage; 

47 â€” the yolk globules of the sixth stage (la 2 -Id 2 ) 
separated from globules of the fourth stage (la 2 -Id 2 ); 

50 â€” the ninth stage of division. 



Figure 40. War-nek* s drawings for his work on the development 
of gastropodan molluscs (development of a slug) 



11 â€” in the place of the lost membrane of the nucleus, 
the oval light spot is seen; 

12â€” the yolk has the form of a globule thickened from 
one side; the light spot is elongated; 

13 â€” in the thickened side of the yolk the beginning of 
the formation of a fissure is seen; the light spot becomes 
more elongated; 

14â€” the fissure twists the yolk diagonally; the light 
spot is extended; 

15 â€” the fissure envelops half of the periphery of the 
yolk; at the ventral pole is the beginning of the formation 
of a fissure in the form of a deepening; 

16 â€” deeper twisting of the yolk, viewed from the dorsal 
pole; 

17 â€” the yolk globules are completely separated from each 
other; the light spot is hardly noticeable; 

26 â€” the nuclei are deprived of membranes and begin to 
extend in a direction perpendicular to the longitudinal pole; 

26a â€” still more distinct changes, the beginning of which 
is illustrated in Drawing 26; 

29aâ€” the stage of four yolk globules. 


On the basis of experiments on ova which had been 
pressed in water, Warnek concluded that the structural 
changes in the nuclei depend upon the change of their 
chemical composition. The chemical changes that appear, 
in Warnek' s opinion, condition also the further transformations in the developing ova. After that when the fissure 
begins in the dorsal side (Figure 40, 14), it passes around 
half the periphery of the yolk; also in the ventral side, 
a deepening appears (Figure 40, 15) and the yolk is twisted 
by the meeting fissures. It acquires first the form of a 
biscuit, and then two united or even completely isolated 
globules (Figure 40, 17) . At the time of division of the 
yolk the light spot (nucleus) divides into two parts, each 
of which at first have caudiform processes, directed to the 
point of contact of the yolk globules (Figure 40, 16) . The 
processes quickly disappear, and the spot becomes spherical. 

These phenomena, according to Warnek, characterize 
the first half of the second stage of division. At this 
time it is not possible to isolate the nuclei by (pressing) 
the yolk globules; from which it must be concluded that the 
nuclei are still deprived of membranes, 

The second half of the second stage begins with the 
dividing globules moving nearer. Between them a noticeable 
cavity, formed from a transparent substance, emerges . Warnek 
considered this transparent substance the product of separation of the yolk globules. At the time the dividing globules 
are moving closer, the contours of the nuclei in them again 
become clear, i.e. the membrane appears (it is clearly 
noticeable in Li-max) by strongly refracting the color of the 
nucleolus. In Lymnaeus stagnalis the nuclei, at first, are 
situated near each other (Figure 39, 22), then separate, 
sink in the depth of the yolk globules, and at the end 
become invisible (Figure 39, 25) . Following the described 
phenomena each of the two yolk globules become pear-shaped. 
This outlines the passage to the third stage of division, 
during which the yolk is divided into four parts. 



THE THIRD STAGE begins with changes in the nuclei, 
acquiring a biscuit or figure-eight shape (Figure 39, 25a); 
in the drawing it is seen that their division is not 
accomplished simultaneously: when one divides, the other keeps the form of a quadrant. The details of these changes 
can be traced only in the transparent ova of slugs. When the 
yolk takes the biscuit form, the nuclei are elongated 
(Figure 40, 26); following this the elongation and twisting 
of the yolk globules themselves takes place (Figure 40, 26a) . 
The twisting in Lymnaeus stagnalis and slugs begins in one 
globule earlier than in the other (Figure 39, 27), however, 
soon after this difference smoothes out. The boundary between 
the globules of division, corresponding to the first fissure, 
is at first straight (Figure 39, 27), and then becomes 
curved (Figure 39, 29, 30), and the yolk globules are situated 
crosswise in two planes. 

The processes taking place in the nuclei were described 
by Warnek as follows: "The membranes of the nuclei disappear, 
the nuclei elongate, take an oval biscuit-shaped form, then 
bulge out, and finally each nucleus from the beginning of 
the division of the yolk globules is divided into three parts. 
From these parts of the nuclei only four are present in the 
globules of the division, and two gradually disappear in 
the fissures between the globules. The four nuclei at first 
have the form of a comet. When the division of the yolk 
globules is finished, the CAUDIFORM processes of the nuclei 
extend and the nuclei again acquire the rounded form" 
(pp. 146 - 147). The three parts into which, according to 
Warnek, each nucleus is divided correspond to the two daughter 
nuclei and to the achromatic figure of mitosis situated 
between them. 

The final step of the third stage is the formation of 
the membranes around the nuclei and subsequent turning of 
the cross-shaped globules of division. Two of them are in 
contact with each other on the dorsal side, and the other 
two on the ventral (Figure 39, 34) . Between the yolk globules 
a rhomboid space appears during which this is especially clear 
in Limax. 



THE FOURTH STAGE . At the beginning of the fourth 
stage the nuclei in the yolk globules of Lymnaeus stagnalis 
again become unnoticeable from outside. During the crushing 
out of ova it is possible, however, to see the changes 
occurring in the nuclei, which, as in the previous stage 
do not take place at the same time. In Figure 39, 36a it is seen that in the two globules of division which have the 
longitudinal form, one nucleus is biscuit-shaped, and the 
other consists of two isolated parts. After that, when the 
nuclei become elongated (in slugs this is seen also in the 
intact globules of division] , each yolk globule stretches 
and becomes pear-shaped. Then the twisting occurs in this 
form so that the newly forming globules are of unequal size, 
each separating a region of one third the size. The four 
smaller globules become displaced and are situated in the 
spaces between the larger two (Figure 39, 40) . "The remarkable 
reciprocal situation of the yolk globules," Warnek wrote, 
"is kept and is repeated in all the following stages; this 
allows one, without ever noticing the further formation of 
the yolk globules, to solve the question, what globules of 
division result from each present globule. During this it 
is necessary to keep in mind the position of the nuclei and 
the relative size of the yolk globules" (p. 153). 

These accurate observations surpassed those investigations of nearly a quarter of a century afterwards. By the 
initiative of A. 0. Kovalevsky, the blastomeres of the 
dividing ova were given individual designations (in letters 
and numbers) , tracing during the process of development the 
fate of each blastomere and its derivatives. Warnek formally 
applied a less suitable and obvious method of designation 
of the globules of division and their descendants. He 
named the blastomeres arising in one or the other stage by 
the number of this stage, keeping for them the same designation also in the following stages of division. During this he 
mentioned that the yolk globules changed from stage to stage, 
so that, for example, during the transfer to the fifth stage 
the globules of the fourth stage were already unequal to the 
globules of the fourth stage at the moment of their formation. 
They decrease in size and are changed by chemical properties 
and internal structure. 



THE FIFTH STAGE. In this stage the formation of 
the new globules of division follows the rules which also 
hold for the following stages of division. Instead of the 
sixteen yolk globules which must be present if each of the 
eight globules of the fourth stage is divided, here only 
twelve globules are found. This can be explained by the fact 
that during the fourth stage only four large globules are divided, and the other four small globules remain unchanged. 
Before the division itself the nucleus of the ventral (large) 
yolk globules become invisible, although on crushing out of 
the ova, it is seen that they elongate, i.e. they are present 
in a condition of division, while the nuclei or the dorsal 
(small) globules are not divided and remain round (Figure 39, 
43) . The dividing yolk globules are stretched and twisted 
in the diagonal direction (Figure 39, 45). The newly arising 
yolk globules (again smaller in size than those which gave 
them the origin) are generally situated by the general rule, 
in the spaces between the large vegetative globules. The 
twelve globules of division present in the fifth stage are 
situated in three rows. The ventral row consists of four 
globules of the third stage, in the dorsal side four yolk 
globules of the fourth stage are present, and between the 
ventral and dorsal globules four newly arising yolk globules 
of the fifth stage are situated. The globules of the upper 
and lower rows stand against each other, and in the spaces 
between the globules of these rows the globules of the middle 
row are present. Concerning the origin of globules of the 
fifth stage from the globules of the third stage it is 
judged by the neighboring situation of their nuclei (Figure 39, 
46a). In the second half of the fifth stage, as in the previous 
one, the smoothing of the surface of all globules of division 
and their nuclei becomes more distinct. Between the yolk 
globules a vesicular light space appears. 



THE SIXTH STAGE. At the beginning of this stage four 
yolk globules of the fourth stage become more convex and 
the globules of the sixth stage separate from them. The 
yolk globules of the fifth stage remain the spaces between 
the globules of the third stage (Figure 39, 47). The total 
number of globules of division in this stage is sixteen. 



THE SEVENTH STAGE is characterized by the three 
divisions of the globules of the third stage, giving rise to 
the four yolk globules of the seventh stage. The total number 
of globules of division is twenty. 



THE EIGHTH STAGE. In this stage the four globules of 
the eighth stage are separated from the yolk globules of the fourth stage. The total number of yolk globules grows to 
twenty- four. 



SUBSEQUENT DIVISION. In each of the following 
stages four yolk globules are formed. As an example the 
ninth stage can be employed. In the given drawing (Figure 39, 
50) the dividing ovum is pressed, so it is possible to see 
a great number of globules of division. In the middle the 
yolk globules of the fourth stage are situated. To these 
last the oldest and large globules of the fourth stage are 
adjoined, moving far aside from each other by pressure. 
To the left of the globules of the third stage the globules 
of the fifth stage are present, obliquely from which the 
youngest globules of the ninth stage are twisted; they are 
situated in the spaces between the globules of the fifth 
and seventh stages. The globules of the seventh stage 
originate also from the globules of the third stage and 
are situated to the right of the last globules. Finally 
between the yolk globules of the fifth, seventh and fourth 
stages the globules of the eighth stage are present, arising 
as a result of a second division of the globules of the 
fourth stage. 

Further Warnek carried out an analogical analysis of 
the fifteenth stage and established the origin of all globules 
present at this moment of division. 

It is instructive to compare the genealogy of blastomeres, 
established by Warnek, in gastropodan molluscs with recent 
data. This comparison shows the complete agreement of Warnek 's 
results with recent data, as seen from the table. In it the 
designations applied by Warnek and the presently applied 
literal numerical designations are given. 

The same comparison is given in Figure 41, where the 
contours of drawings 40, 45, 47 and 50 of Warnek are repeated 
and the data of the recent designations of blastomeres and 
their designations by Warnek are compared. 

There is no doubt that Warnek completely and distinctly 
chose to follow the fate of the separated blastomeres ("yolk 
globules") and the participation of their descendants in the 
formation of organs of the developing animal. Selecting an irreproachable method by which this problem can be solved, 
Warnek in the first published investigations on the 
embryology of gastropodan molluscs described the first 
period of development up to the formation of the spherical 
multicellular stage, i.e. the blastula. Only at the end did 
he briefly mention the following period, when "some yolk 
globules share in the formation of first internal organ â€” 
the yolk sac." This first internal organ, the yolk sac, is 
of course nothing other than the endoderm of the embryo. 

Concerning some details of division, Warnek noted that 
during the division of the nuclei of the yolk globules the 
nucleolus appears earlier than the nucleus when divided into 
two parts, therefore it is possible to find nuclei with two 
nucleoli. Warnek never saw the process of division of the 
nucleolus itself. The division of the nucleus in his experience 
was always accomplished by one plan, which in the early 
stages of divisions was the same as in the subsequent development of the embryo. This division in all conditions takes 
place after the stage of stretching of the nucleus, which 
then acquires the shape of a biscuit and a figure eight and 
is finally transformed into two separate nuclei. 

The globules of division Warnek identified as the 
elementary organs, i.e. cells, and considered that their 
multiplication, beginning at the time of division, continued 
throughout the period of development and even through the 
entire life of the animal. 

All the activity of the developing embryo and the 
animal forming from it is, in Warnek' s opinion, the result 
of that primary influence which the ovum ["yolk mass") 
is subjected to by the semen. "This influence," Warnek 
wrote, "has a purely chemical nature; therefore the explanation for this is still obscure for us; the vital phenomena must 
be given by physicists and chemists" (p . 168). "The effective 
element in the organism," Warnek continued, 

is the material; this same material influences also 
outside the organism. If we explain this activity by 
chemical and physical powers, then there is no reason 
to deny the activity of these powers in the organism 
as well. Although these powers still cannot be completely explained, we do not possess the right 
to discard them and resort to the help of this power, 
which exists only in the imagination. Can we explain 
the phenomenon of crystallization? Why does sodium 
chloride always crystallize in the form of a cube, 
and pure carbon in the form of an octahedron? Is 
not the formation of globules of division, from the 
point of view of form, a kind of crystallization of 
organic matter? The successes of organic chemistry 
belong to us, because the processes accomplished 
during nutrition, respiration, and excretion are 
more satisfactorily explained by means of physics and 
chemistry than by means of a special vital power. This 
power has retreated into the dark field of our 
knowledge about the functions of brain and nerves and 
still dominates in the sphere of embryology. However, 
new histological directions make the study of the 
vital power even more unsteady in this sphere, so we 
are not far from the time when chemistry will 
completely exclude it from there as well. The concept 
of vital power must remain as a reminder of our 
previous ignorance. Only quite recently the influence 
of semen on the yolk was called dynamical; this 
expression shows only that the phenomena of fertilization could not be explained, (p. 170) 

In these words Warnek exhibited the materialistic world 
view with complete clarity. He decisively objected against 
the dull idea of the vital power for the explanation of 
phenomena of organic life. The only way in which this 
explanation could be achieved Warnek considered to be the 
physico-chemical investigations of vital phenomena. Regarding 
fertilization as a chemical process, Warnek thought that 
the subsequent transformations of the dividing ovum have as 
their source continuous chemical changes. Of course, Warnek' s 
materialism has a mechanical character, but it is not 
excessively simplified vulgar materialism. 

Warnek 's embryological opinions are expressed in his 
theses, the most important being: 

The yolk mass after fertilization undergoes chemical 
changes, therefore the fertilization itself must be regarded 
as a chemical process. It causes changes in the unfertilized 
ovum which are necessary for further development of the embryo 


Table 
The Genealogy of Blastomeres 

Comparison of Warnek's designationsâ€” stage numbers (in parentheses! in comparison with the recent literal-numerical. Cln 
the square brackets the designation of the resulting blastomeres of the previous stage are repeated.) 



Key: 



1 . Stage of division by Warnek 


5. The sixth 


2. The third 


6 . The seventh 


3. The fourth 


7. The eighth. 


4. The fifth 


Figure 41. Comparison of Warnek's designations of blastcraeres 
(stage numbers) with the recent literal-numerical 
designations of blastomeres . These and the other 
designations are put in the contours of Warnek's 
drawings . 



During the development of the embryo, further changes of 
chemical processes take place. 

The gastropodan molluscs are characterized by complete 
division. 

The globules of division may be considered true cells. 

In each stage of a division process four yolk globules 
are formed, i.e. the division proceeds not in geometrical, 
but in arithmetical progression. 

Beginning with the third stage, the globules of division 
have unlike sizes. 

Warnek's work produced a new page in embryology, 
directing the investigations of the history of individual 
development towards the study of subsequent changes of the 
fertilized ovum and the forming from it of blastomeres, and 
towards a study of the fate of the separate blastomeres and 
their descendants during the subsequent formation of the 
embryo. In this sense Warnek's investigation foreshadowed 
the works of A. 0. Kovalevsky and his countless followers 
who were studying either descriptively or experimentally the 
transformation of the elementary organs of the dividing ovum 
the blastomeres into systems of organs of the forming organism 

N. A. Warnek was for a long time undeservedly forgotten. 
His classical work is rarely cited and not always mentioned 
even in the embryological summaries and textbooks, although 
he unquestionably deserved a place of honor in the history 
of Russian and world embryology. 



The investigations of Grube, Nordmann and Warnek were 
monographical descriptions of the embryological development 
of certain representatives of the invertebrates. These works, 
with all their significance, did not answer, however, the 
requirements of comparison of the phenomena of development 
in different types of animals. The first attempt to include a 
wide number of invertebrates in embryological investigations 
was done by A. Krohn, whose services in this sphere are much 
undervalued. Krohn was so thoroughly forgotten that his 
name was not mentioned either in the encyclopaedias or in 
the biographical reference books. 30 The following circumstance 
is sufficient to attract the attention of historians of Russian 
science to Krohn. 




30. For help given in researching biographical and bibliographical data about Krohn, the author thanks the 
biological section of Saltykov-Sedrin Gos. Publichnaya 
library in Leningrad, especially librarian V. L. Levin 



During the first committee discussion of Baer's prize of 
the Russian Academy of Science in 1867, considering possible candidates for the prize, the following was stated: "If the 
matter concerned the crowning of previous scientific works, 
then the committee does not doubt that the prize belongs to 
one of our compatriots, Krohn, who was born in Petersburg. 
For many years from the fertile shores of the southern seas 
he collected a rich material which he investigated for the 
development of different animal forms. His investigation 
resulted in many excellent works which deserve respect from 
the scientists of all countries. However, the competition 
was to take under consideration only the works of the last 
three years. "31 

The absence of biographical information about Krohn is 
compensated for by some bibliographical data. It is established that Krohn published no less than eighty works, 32 
including some small monographs. Many of his publications 
were accompanied by indications of the time and place of 
performance of the corresponding work. With these indications 
one can form opinions about the life of Krohn, who spent no 
less than thirty years in travel with the aim of scientific 
investigations, zoological and embryological . 




31. "Extract from the report of the committee on the discussion 
of the prize of secret adviser K. M. Baer, read in public 
meeting of the Academy of Science on February 17, 1867 by 
Academician Ovsyannikov, " NATURALIST (1867), Vol. 4, 
No. 7-9, pp. 98 - 104; No. 10 - 12, pp. 146 - 148. The 
extract cited is on p. 99. 

32. The list of publications by A. Krohn is presented in the 
following : 

1. CATALOGUE OF SCIENTIFIC PAPERS, compiled by 
the Royal Society of London (Vol. Ill, 1869- Vol. VIII, 
1879) ; 

2. w. Engelmann, BIBLIOTHECA HISTORICO-NATURALIS, 
Suppl . Band: J. V. Carus und w. Engelmann, BIBLIOTHECA 
Z00L0GICA: VERZEICHNIS DER SCHRIFTEN UBER 
Z00L0GIE, WELC'HE IN DEN PERIODISCHEN WERKEN 
ENTHALTEN VOR JAHR 1846 - 1860 SELBSTANDIG 
ERSCHIENEN SIND, Ed. I - II (Leipzig, 1861), 2144 pp. 

33. Information on the dates of Krohn' s birth and death are 
taken from the brief bibliographical catalogue published by 
the Library of Congress in Washington. 


August David Krohn was born in Petersburg in 1803.33 
Concerning his birth and student years of study of Krohn we could not discoyer any information. One of his early works 
was produced in Vienna (1836); of the next work there is a 
memorandum, Petersburg (1 83 7) , Later on Krohn was in 
Heidelberg (18:59], and from 1840 he worked nearly continuously on the shores of the Mediterranean Sea and the islands 
of the Atlantic Ocean. In 1840 and later he was in Naples; 
from the autumn of 1844 to the spring of 1845 in Messina; 1848 
in Nice; 1850 in Naples. In the beginning of 1835 and the 
winters of 1853/54 and 1856/57 he again was in Messina; in 
the winter 1855/56 and the spring and summer of 1865 he 
worked in Funchal (in Madeira) and in Santa Cruz (Tenerif e) . 
In December 1860, in May 1861 and in 1867 Krohn was in Nice, 
and in the first half of the year 1869 he was in Naples. In 
the intervals between travelling he lived in Paris (winter of 
1851/52, spring of 1857) and in Bonn (summer months of 1851, 
1853, 1855, 1857 and 1859, winters of 1859/60, 1864/65 and 
1856/66, and also the second half of the year 1869) . 

Concerning the last twenty years of Krohn 1 s long life of 
eighty-eight years there is again no information. 

During his travels Krohn maintained contact with his 
country, as seen by the report of the conference of the 
Academy of Science in Petersburg, which presents the 
following records: 

"Mr. August Krohn is a doctor who is famous for his works 
on anatomy and physiology. He has sent the Academy a significant collection of invertebrate sea animals which was collected 
by him near Naples, which, in quantity and quality, deserve 
the thanks of the Academy" (Report from December 16, 1842) . 

"Dr. Krohn sent again, as a gift to the Academy Museum, 
two new collections, about one hundred species of fish, 
crustaceans and others" (Report of September 13, 1844). 

On the 7th of November 1855 Krohn was recommended as 
candidate for corresponding member of the Academy of Science 
in Petersburg, hit was not elected. The biographical data 
given during this presentation shows only that he was born in 
Petersburg, lived abroad, and wrote about thirty valuable works 
dedicated to molluscs (IPaludina, Phyllirhoe and oephalpoda) , 

worms (SipunculuSj Syllisj Aloiopa) , and tunicates 
{Doliolum) , 34 

During his travels Krohn entered into friendly relations 
with many great zoologists of the time, such as Johannes 
Muller, M. Sars, and Delle-Kyaie, and also the young 
investigators A. Kolliker, 35 k. Gegenbaur, 36 an( j a. Schneider. 
Johannes Muller (1801 - 1851) was a great German zoologist, 
embryologist and physiologist. For a long time he was the 
editor of the widely distributed journal ARCHIV FUR 
ANATOMIE, PHYSIOLOGIE UND WISSENSCHAFTLICHE MEDIZIN, 
in which Krohn published about thirty articles. Part of this 
information Krohn sent to the editor of the journal in the 
form of letters, containing information about his last works, 
and Muller published them in his ARCHIV which he sometimes 
accompanied by remarks and additions, always with a friendly 
and positive tone. In those remarks of Mill Her' s, discussions 
can frequently be found revealing his high regard of Krohn' s 
scientific activity. 

Krohn willingly related his observations to the zoologists 
who were working at the same time with him along the sea coast 
C133) . His objective was to verify his data and to confirm 
their authenticity, and equally to help also the beginning 
investigators. His description of the planula hydromedusa 
Cladonema, Krohn accompanied with the remark that he showed 
them to Sars and Gegenbaur, thus certifying the accuracy of 
his observations. 



34. Archives AN SSSR, fund 2, inventory 17, No. 6. The author 
is deeply grateful to B. E. Raikov. On a commission from 
him, extracts of reports of conferences of the Academy 

of Science were carried out here and an Archives Certificate 
received. 

35. "For accurate information about the structure of marginal 
bodies in medusa," Krohn wrote, "I am grateful to my young 
friend Kolliker from Zurich" (A. Krohn, "Einige Bemerkungen 
und Beobachtungen uber die Geschlechtsverhaltnisse bei den 
Sertularinen," ARCH. ANAT . , PHYSIOL. (1843), pp. 174 181} . Albert Kolliker was a well-known histologist and 
embryologist . 

36. Karl Gegenbaur (1826 - 1903) later became a famous 
comparative anatomist and embryologist. 

Anton Schneider (1831 - 1890) was a famous German 
zoologist, who in a work on the development of the mollusc 
Phyllirhoe bucephalumS? warmly mentioned "the repeated 
friendly directions" which he received from Krohn not only 
during the observations taken for that work, hit also 
throughout the time of their presence together in Messina 
in the spring of 1858. Significantly later, in 1867, Krohn 
met with Schneider in Nice; Schneider was interested there 
in larvae of any polychaetes which were covered with peculiar 
porous membrane. Schneider found that these larvae were well 
known to Krohn. In addition, as Schneider wrote, "Krohn, 
with his characteristic generosity, gave me the relevant 
pages of his journal that I might use the information 
contained in it at my discretion, expressing the hope that 
I could trace the further development of these larvae" (p. 498, 
footnote) . 

Other works reveal that Krohn did not intend to publish 
his materials on the development of this polychaete, and 
agreed that Schneider would do this himself. Schneider wrote 
the work, its first part (description of the early stages) 
containing his own materials, and the second the results of 
Krohn 's observations on subsequent development. The work was 
published under the names of both authors, Krohn 's name in 
first place. 38 

Helping to increase his material, Krohn showed at the same 
time extreme punctiliousness in relation to the strange data. 
This is illustrated by Krohn' s following remark on one of the 
early works about the structure of the nervous system in the 
echinoderms,39 as he sought to eliminating all shades of 
suspicion of incorrectness in relation to the published data 
of other investigators: 



37. A. Schneider, "Uber die Entwickelung der PHYLLIRHOE 
BUCEPHALUM," ARCH. ANAT . , PHYSIOL. (1859), 
pp. 35 - 37. 

38. A. Krohn und A. Schneider, "Uber Annelidlarven mit porosen 
Hullen," ARCH. ANAT., PHYSIOL. (1867), pp. 498 - 508. 

39. A. Krohn, "Uber die Anordnung des Nervensystems der 
Echiniden und Holothurien im Allgemeinen," ARCH. ANAT . , 
PHYSIOL. (1841), pp. 1 - 13. 



After I finished my observations on the nervous 
system of echinoderras and reported all the existed 
to Mr. Delle-Kyaie, I learned from this scientist 
that Mr. Van Beneden, a year before, had already 
discovered traces of the nervous system in echinus, 
information about which had appeared in L'INSTITUT. 
Because I could never get the proper issue of this 
journal, I should not be birred for not mentioning 
Van Beneden' s observations . (p. 7) 

In all his works Krohn, with exceptional honesty and 
modesty, mentioned the results of the work of his predecessors, not fearing to recognize the superiority of foreign 
observations over his own. 40 

No information was kept on personal events in Krohn 's 
life. He was not connected in his work either with scientific 
institutions or with universities. His life as a travellernaturalist hardly assisted the acquisition of a family of 
his own. To his relatives Krohn superficially referred in a 
letter to Johannes Muller:41 "After eight months absence, 
during which I spent April and May in Santa Cruz in Tenerife, 
I returned to Europe. The immediate cause for this was a 
forthcoming meeting with close relatives, whom I had not 
seen for some years" (p. 515). 

In the first period of scientific activity (up to 1846) 
Krohn' s scientific interests were concentrated in the anatomy 
of vertebrates (fish, amphibia, birds) and invertebrates 
(coelenterates, annelids, arachnids, chaetognatha, molluscs, 
bryozoa, crustaceans, echinoderms, tunicates) . Incidentally 
to his morphological investigation, Krohn found parasites 
in the venous sinuses of cuttlefish (apparently diciemid) and 
described new species of pteropod and cephalopod molluscs. 

Zootomical and zoological investigations were continued by 
Krohn, investigating the structure of protozoa, siphonophora, 



40. Concerning the budding of the complex ascidian, Krohn 
wrote: "Mechnikov, with greater success than I, has 
traced the gradual development of buds" (Krohn, "Uber 
die Fortpflanzungsverhaltnisse bei den Botrylliden, " 
ARCH. NATURGESCH., 35 (1869), pp. 190 - 196). 

41. ARCH. ANAT., PHYSIOL. (1856), pp. 515 - 522. 


annelids, sea spiders, and arachnids, and describing new 
species of annelids, chaetopods, and gastropodan molluscs. 

There is much authoritative evidence on the accuracy of 
Krohn's observations and morphological descriptions. These 
are sufficient to justify the opinion of Kovalevsky, 42 who 
was shared in the discussion of the nature of the so-called 
"ventral saddle" of sagitta. Krohn in 184443 considered 
this formation to be due to the nervous ganglion. Later, 
W. Busch44 corrected Krohn's opinion, and, in spite of the 
latter 1 s objection, Busch shared this point of view with 
Keferstein, R. Leuckart, Pagenstecher, and K. Gegenbaur. 
Keferstein did not agree with Krohn, but he gave credit to 
his anatomical investigations: "Krohn, as it is known, 
related this saddle's very great size to the nervous ganglion. 
I, together with Busch, do not doubt in that this excellent 
investigator was in the present question mistaken. "45 

Kovalevsky, again investigating the anatomical structure 
of sagitta, strongly supported Krohn against the above mentioned 
authoritative zoologists. "I am against the new investigators," 
Kovalevsky wrote, "in considering Krohn correct concerning 
the ventral ganglion, and I hope to convince my readers of 
this also" (p. 135). "The ventral ganglion," he continued, 
"has the form of a long oval or quadrangular body with four 
large nervous trunks, from which two on the anterior end 
continue to the brain or the cephalic ganglion, connecting, 
as Krohn showed correctly, with the lateral nerves of the 
cephalic ganglion" (p. 136) . 



42. A. O. Kovalevsky, "Embryologicheskie issledovaniya chervei 
i chlenistonogikh" (Embryological investigations of worms 
and arthropods) (1871), IZBRANNYE RABOTY (Izd. AN SSSR, 
1951) , pp. 123 - 266. 

43. A. Krohn, ANATOMISCH-PHYSIOLOGISCHE BEOBACHTUNGEN 
OBER DIE SAGITTA B I PUNCTATA (Hamburg, 1844), 16 pp. 
This work was published a year later in French (ANN. SC . 
NAT., 3 Ser., Zool., 3 (1845), pp. 102 - 116) and in 
English (ANN. NAT. HIST. 16 (1845), pp. 289 - 304). 

44. w. Busch, BEOBACHTUNGEN UBER AN ATOM IE UND 
ENTWICKELUNG EINIGER WIRBELLOSEN SEETHIERE 
(Berlin, 1851), viii + 143 pp. 

45. Cited in the article by A. O. Kovalevsky. 



From 1846, Krohn frequently turned to the study of the 
phenomena of reproduction and development of different 
invertebrates - coelenterates, worms, molluscs, crustaceans, 
and mainly the echinoderms and tunicates. 

The development of coelenterates is described in the 
following words â€” first, concerning the hydromedusa Cladonema 
and its\ development from the polyp Stauridium^ : the polyp 
forms buds from which the medusae are formed. Similar to 
oceanids in these medusae, as in Ooeanidae , in the walls of 
the stomach the sexual products develop. If mature males and 
females are situated in separate vessels, then after a short 
time on the bottom and walls of the latter ova can be seen, 
covered by a closely adjacent yolk membrane. That these ova 
are fertilized, Krohn judged by the absence of any embryonic 
vesicle and embryonic spot (nucleus and nucleolus) . Krohn 
mentioned I later the process of ovum division, though not 
describing it in detail, and referring to the fact that this 
process was observed already by Dujardin, who did not, however, 
evaluate/ its significance. Within two days after fertilization 
the formed larva is seen in the egg, which later on leaves the 
ovum membrane and swims with the help of cilia. The larva 
was characterized by a light superficial layer and included 
a dark, probably hollow nucleus (Figure 42, A). By its structure 
the larvae of Cladonema are similar with the young of higher 
organisms such as planulae (Aurelia, Cyanea, Cephea) . After 
two to five days the planula Cladonema becomes rounded, 
situated on the bottom, loses its cilia, and is transformed 
into a disk, not changing its internal structure. In the 
middle of the disk appears a round, hollow hillock, which grows 
later into a cylindrical process, composed of two layers 
present in planulae. On the upper end of the cylinder (rudiment 
of the polyp) four hillocks form, corresponding to the 
external ends of the future antennae. Already at this stage 
the first stinging capsules are seen (Figure 42, B) . Thus, 
Krohn concluded that "Stauridium resulted from the budding 
of the medusa Cladonema, which reproduced by ova; the young 
developing from the ovum is again transformed into the form 
of a polyp. The subsequent change of heteromorphic generation, 
from which more highly organized medusa develop must be 
regarded as a generic form and is considered, consequently, 
factually proved" (pp. 425 - 426).



46. A. Krohn, "Uber die Brut des Cladonema radium und deren 
Entwickelung zum Stauridium," ARCH. ANAT., PHYSIOL. 
1853, pp. 420 - 426. 


 

A preliminary report on these observations Krohn included 
in a letter addressed to Miiller.47 




Figure 42. Planula hydromedusa Cladonema (A) and the 
polyp developed from it Stauridium (B) by 
Krohn . 



Two years later Krohn published a report about the 
structure of the early stages of development of the medusa 
Pelagic. noctitucaA^ At first he found near Messina invertebrate 
medusae similar to ephyra scyphomedusa, separating from "polypform helminths" (scyphistoma) , and he found also earlier larvae. 



47. A. Krohn, "Uber einige niedere. Thiere. Brief liche 
Mitteilung a. d. Herausgeber, " ARCH . ANAT . , PHYSIOL 
C1853) , pp. 137 - 141. 

48. A. Krohn, "Uber die friihesten Entwickelungsstufen der Pelagia nootiluca," 
pp. 491 - 497. ARCH. ANAT., PHYSIOL. (1855), 



The supposition that they were stages of development of 
Pelagia noctiluca was completely confirmed. After many 
unsuccessful attempts, Krohn could carry out artificial 
insemination. As a result of division ,larvae formed having a 
cylindrical, usually stretched form (Tigure 43, A). The end 
of the larvae (a) which is directed forward while swimming 
is rounded, and the other (b) is chipped off. The surface of 
the larva is covered by short cilia. In the blunt end occurs a 
depression with an extremely small, round orifice is seen. 
This orifice is the mouth, which leads to the round, clearly 
outlined cavity of the stomach (c) , occupying the posterior 
third of the body. Tne mouth and stomach, in Krohn 's words, 
are already clearly differentiated already in the natural forms, 
but it is still a non-hatched embryo. However in the present 
stage the stomach is shorter and more rounded than in the 
free larvae. 

Mechnikov rated highly this discovery by Krohn. In 
his monograph EMBRYOLOGICAL INVESTIGATIONS ON MEDUSA 
(1866), 49 he noticed the weak interest in embryology by the 
zoologists of the mid-nineteenth century. He wrote: 

Even important generalizations, such as the similarity 
between the two layers of coelenterates and the 
embryonic layers of the higher animals, emphasized by 
Huxley, and significant facts, such as Krohn' s discovery 
of the formation of a stomach in pelagia by a 
stretching of the blastomeres, remained without 
attention and in a lower plane, (p. 284) 

The correctness of these observations by Krohn was later 
confirmed by Kovalevsky and Mechnikov. In this work also, 
Krohn reported one important discovery: "On the contrary to 
Medusa aurita and other above-named medusae, "50 he wrote, 
"Pelagia noctiluoa develops without the generation of 
helminths" (p. 469). Krohn could trace how the swimming planula Velagia, while not settling on the bottom and not 
transformed into scyphistomae, forms on the edges of the 
mouth orifice processes, later becoming part of Ephyra 
(Figure 43, B and C) with marginal sensory bodies. The 
citation of this discovery can be found either in later 
investigators of the embryology of medusae (for example, 
KovalevskySl and Mechnikov (134)], or in textbooks. 52



49. Cited in I. I. Mechnikov, IZBRANNYE BIOLOGICHESKIE 
PROIZVEDENIYA (1950), pp. 271 - 472. 

50. Krohn compared the development of Pelagia with the 
development of Medusa, Cyanea, Chrysaora, Cephea, and 
Cassiopea . 





In 1861, during his residence in Nice, Krohn observed 
the reproduction and development of hydromedusa Eleutheria , 53 
The ova arise between ecto- and endoderm and there they 
develop into the larval stage of larva. Ectoderm, covering 
the embryonic chambers, swells into hillocks, which subsequently break and release young. The larvae are considered 
typical planulae and are subjected to the same transformation 
as in the planula Cladonema. 

The budding occurs not only in asexual, but also in 
completely differentiated bisexually related individuals. 
Krohn described the process of budding, and noticed that the 
budding begins in very young individuals, which are still not 
completely separated from the maternal individual. 



51. A. O. Kovalevsky, "Observations on the development 

of Coelenterata," IZV. OBSHCH . LYUBIT. ESTESTV. 
ANTROP. I ETNOGRAFII, 10 (1874), vyp. 2, pp. 1 pp. 1 - 36. To the work of Krohn there is reference 
on p. 7. ^ 

52. K. N. Davydov, TRAITE D 'EMBRYOLOGIE COMPARES 
DES INVERTEBRES (1928), p. 78. The drawings given 
by Davydov (Figure 36) illustrating the development 
of Velagia were taken by him from the work of Delap, 
published more than fifty years after Krohn' s. They 
are not a bit better than Krohn ' s drawings . 

53. A. Krohn, "Beobachtungen iiber den Bau und die 
Fortpf lanzung der Eleutheria Quatref. , " ARCH . 
NATURG., 27 (1861), 1, pp. 157 - 170. 



Figure 43. Later stages of development of medusa Pelagia 
noot-iluca (by Krohn) . 

a â€” anterior; bâ€” -posterior end of the body; 
stomach. 



The development of worms54 Krohn described in many 
separate reports. In 1851 he wrote an article on the reproduction and larval stages of "gefirei," 55 He established 
the fact of dioecious Phasoolosoma and described the structure 
of mature ova of Sipunculus nudus . Attempts at artificial 
insemination of these ova were unsuccessful, and Krohn had 
to be satisfied with the study of the larvae of Sipunculus 
caught in plankton, whose description constituted the final 
part of the work. Two small remarks by Krohn are concerned 
with the vegetative reproduction of the annulated worm Syllis 
and Autolytus . 56 He found in them this change of sexual and 
asexual reproduction, which permitted comparison with true 
alternation of generations. Many years later 57 Krohn again 
turned to the study of reproduction in sillids, describing 
the new viviparous species of polychaeta of this genus. 



54 . Of the types of worms known in the mid- nineteenth century, many were distinguished later on in independent 
groups of forms, including phoronids and chaetognaths , 
and are so listed. 





Other of Krohn 's reports separate information about the 
development of nemertineans, phronids, and chaetognaths . 
The description of larvae and partial transformation of the 
first two forms constitute the contents of a special 
article^ (135) . On the question of the development of 
nemertineans inside pilidium, Krohn inclined to the opinion 
that pilidium is considered a helminth, giving origin to 
worm-shaped sexual generation. Actinotrocha, in Krohn' s 
opinion, is a larval stage of any worm, tentatively relating 
to echiuroids. The process itself of the transformation of 
actinotrocha he did not observe and noted only the disappearance of the larval organs and the concentration of antennae 
in the circumoral corona. 

In his excellent investigations on the structure of 
chaetognaths, Krohn added the study of their development. 



55. A. Krohn, "Uber die Larve des Sipunoulus nudus nebst 
vorausgeschikten Bemerkungen liber die Sexualverhaltnisse 
der Sipunculiden," ARCH. ANAT . , PHYSIOL. (1851), 
pp. 368 - 379. 

56. A. Krohn, "liber die Erscheinungen bei der Fortpflanzung 
von Syllis prolifera und Autolytus proKfer," ARCH. 
NATURG., 18, 1 (1852), pp. 66 - 76; "Uber die Sprossling 
von Autolytus prolifer Gr.," ARCH. ANAT., PHYSIOL. 

(1855) , pp. 489 - 490. 

57. A. Krohn, "Uber eine lebendiggebarende Syllisart," ARCH. 
NATURG., 35 (1869), pp. 197 - 200. 

58. A. Krohn, "Uber Pilidium und Actinotrocha," ARCH. ANAT., 
PHYSIOL. (1858), pp. 289 - 301. 


In a letter to Johannes Miiller sent on February 2, 1853 
from Messina, 59 he reported: "I have at the same time studied 
the development of sagitta. What Darwin has said about it 
relates to the development of any fish" (p. 141) (136). 
Krohn did not publish any further special work on the 
development of sagitta. 

Of Krohn' s two articles on Cirripedia, one concerns 
the structure of the cement gland of Lepas anatifera and 
Conohoderma virgata and the anatomy of the female genital 
system of Lepas and Balanus trintennabulwn . The other^O 
includes some data about larval development. Krohn described 
the intermediate stage between the young larva which is 
similar to the nauplius of the copepods and the late cirripesshaped larva. The work is illustrated with graphs of very 
young larva of cirripeds, and also larvae of Balanus species 
and Lepas antifera. 

The development of gastropodan molluscs (pteropods and 
heteropods) was the subject of four reports, the last with 
the character of a detailed monograph. ^1 in these investigations his main attention is given to a detailed description 
of larvae of pteropods (subclass opisthobranchia) : Cymbulia 
Peronii, Tiedemannia neapolitana, Gastropteron Meokelii 3 and 
also the larvae of carinate molluscs (subclass prosobranchia) 
Pterotraohea (two species) , Carinaria mediterranean and 
Fir-iolides (Figure 44) . 



59. ARCH. ANAT., PHYSIOL. (1853), p. 137. 

60. A. Krohn, "Beobachtungen iiber die Entwickelung der 
Cirripedien," ARCH. NATURG . , 26, 1 (1860), pp. 1 - 8. 

61. A. Krohn, "Beobachtungen aus der Entwickelungsgeschichte 
der Pteropoden, Heteropoden, und Echinodermen. Brief 1. 
Mitt. a. d. Herausgeb.," ARCH. ANAT., PHYSIOL. 
(1856), pp. 515 - 522; "Beitrage zur Entwickelungsgeschichte der Pteropoden und Heteropoden," ibid . (1857), 
pp. 459 - 468; "Uber die Schale und die Larven des 
Gastropteron MeckelM," ARCH. NATURG., 26, 1 (1860), 
pp. 64 - 68; BEITRAGE ZUR ENTWICKELUNGSGESCHICHTE 
DER PTEROPODEN UND HETEROPODEN (Lepizig, 1860), 

46 pp. 



Figure 44. A-Larva Ctio from the dorsal side with not 
completely straightened fans of sailing; 
B^â€” extracted from ovum larva F-ivioZides , 
view from above (by Krohn) . 



The accuracy of Krohn's observations concerning the 
structure of the larvae of gastropodan molluscs is noted in 
recent books on comparative embryology, For example, 
K. N. Davydov wrote that "Already, long ago, zoologists turned 
their attention to the development of gastropodan molluscs, 
and Krohn and Nordmann left for us memoirs which even at 
present do not lose their significance" (p. 625) .62 ^ n( j 
elsewhere: "It is known that the classical case of heteropods 
(_Firzolid.es) was described by Krohn in 1860" (p. 651). 

A significant place in Krohn's scientific heritage is 
occupied by his investigations on the development of 
echinoderms; they were described in no less than ten special 
reports and transitional notes in works describing other 
subjects. His systematic study of the embryology of echinoderms 
Krohn began in 1848 at the time of his three-month stay in 
Nice. In February-April he experimented with the artificial 
insemination of the ova of the echinoid Echinus lividus . His 
observations were compared with data published shortly before 
by Derbes, 63 w ho also studied the development of ichinus 
brevispinosus . 

The mature ovum of Echinus lividus, according to Krohn, 
is covered by membrane (chorion) and composed of yolk (this 
term Krohn called the ooplasm with nutritional inclusions), 
with an embryonic vesicle (nucleus) and embryonic spot 
(nucleolus) . Derbes assertion that the embryonic vesicle 
disappears in the mature ova up to fertilization, Krohn 
considered the result of Derbes' insufficiently thorough 
investigation. Within approximately half an hour after fertilization of the ova, the, membrane is separated from the yolk, 
a phenomenon, which Krohn explained by processes of endosmosis 
and exosmosis: "the ovum membrane absorbs the fluid from the 
surroundings and again gives it, to its internal surface" 
(p. 6). After fertilization the embryonic vesicle and the 
embryonic spot are no longer seen. In the place of the ovum 
nucleus, not far from the surface of the ovum, Krohn saw an 
empty vesicle. With Baer and Derbes, he considered this vesicle 
the nucleus of the fertilized ovum and suggested that the 
process of yolk division can begin only after the appearance 
of this nucleus. Within three to four hours after fertilization the division of the nucleus begins. Referring to Baer's 
excellent observations, Krohn left out the description of 
the initial processes of the division. In the conclusion of 
the monograph he noted that the result of the yolk division 
is the formation of cells, from which the body of the embryo 
is formed, because the globules of division are unnoticeably 
transformed into cells of the developing larva. The evidence 
of this transformation is found in the contents of the cells; 
"numerous molecules inside the last substance are nothing 
but yolk granules, from which the division globules are 
previously formed" (p. 29). "In the same genetic relation, 
the nuclei of the cells are related to the vesicular nuclei 
of the division globules" (p. 30).



62. Davydov, TPAITE D ' EMBRYOLOGIE . 

63. Derbes, "Observations sur le mecanisme et les 
phenomenes qui accompagnent la formation de l'embryon 
chez l'Oursin comestible," ANN. SC . NAT., 3 Ser., 

8 (1847) , pp. 80 - 98. 

64. a. Krohn, BEITRAG ZUR ENTWICKELUNGSGESCHICHTE DER 
SEEIGELLARVEN (Heidelberg, 1849), 36 pp. 

 

Krohn prefaced the characteristic of the fully formed 
echinus larva with the development of the larva, suggesting 
that knowledge about the final development must help the 
understanding of the phenomena leading to it. The external 
form of the pluteus, the structure of its skeleton and the 
digestive system are represented in the drawings and are 
described as follows. The formed larva, within eleven days 
after the fertilization, is pear-shaped (Figure 45, F) ; it 
possesses two pairs of limbs: the short ee and the long dd. 
On the convex side, facing the mouth Â£, and situated between 
the limbs, the anal opening c^ is present, although frequently 
closed, which is why Johannes Miiller missed it in the larvae 
of ophiuroids and echinus. The bilateral symmetry of the larva 
is absolutely clear and is expressed in the pairing of limbs 
and the calciferous skeleton carrying their branches, in the 
situation of the mouth and the anus in the plane of symmetry. 
Inside the larva is a cavity extending to the end of the limbs; 
the digestive tract lies in this cavity, surrounded by loose 
fibrous tissue. The larval surface is covered with skin; the 
last is formed from twinkling cells, in each of which there 
is a nucleus with nucleolus. The calciferous skeleton consists 
of four pairs of toothed branches (Figure 45, G) , the longest 
branches gg_ are club-shaped, the next pair is found in the 
long limbs hh, another pair in the short limbs kk, and the 
last pair H_ is situated across the longitudinal axis of the 
larva. 


The digestive canal consists of three parts, the anterior 
(the pharynx) , the middle (stomach) and the posterior 
(intestines), all covered by cells with cilia, similar to 
the cells of the skin. The digestive canal is attached to the 
body cavity by fibrous tissue; it forms a network united with 
the internal surface of the skin. In the junctions of the 
network, nucleus-like formations are situated; they are 
numerous also under the skin, in the neighborhood of the 
calciferous skeleton. Krohn noted Johannes Miiller's error 
when he assumed them to be strong fibers instead of nerves, 
and the nuclei of fibrous tissue instead of nerve ganglia. 
The larva swims with its limbs, mouth opening forward; its 
movement is carried out by the activity of the cutaneous 
twinkling cilia, which also drive into the mouth food particles 
suspended in the water. 

The formation of the larva described here takes place as 
follows. After the division is completed, the young spherical 
larva rotates in the ovum membranes with the help of the 
long cilia which cover all the surface of its body. In the 
larva at this time can be observed a closed central cavity 
and cover, which can be differentiated, forming the wall of 
the latter. Krohn erroneously considered that the wall of 
the body consists of many layers of cells. Within a day after 
fertilization, when the number of the cover cells becomes 
sufficiently large, the ovum membrane is torn and the larva 
begin to swim. Soon after hatching it acquires its ovoid form 
(Figure 45, A); during swimming its narrow end is directed 
forward. 

In the body cavity of the larva, from the side of the 
blunt end, an accumulation of closely situated dark bodies, 
similar to the nuclei, is seen. The number of these nuclei 
at first is small, but it quickly increases so that they are 
found filling half of the body cavity of the larva, toward 
its blunt end (Figure 45, A,e) . Later, nuclei are separated 
from each other and distributed evenly in the body cavity, and 
then partially accumulate not far from the rudiments of 
the calciferous skeleton. After this, the nuclei become angular 
or fusiform and begin their transformation into the fiber of 
the reticular tissue which strings the body cavity and holds 
the digestive tract. 



Figure 45. Development of echinus, Echinus liwidus (by Krohn) . 

A â€” larva shortly after hatching: a â€” blunt pole; b â€” sharp pole; 
c, c â€” cover; d â€” central cavity; e â€” accumulation of "nucleuslike formations" from which the fibrous tissue is later 
formed. 

B â€” "ideal sketch" of the larva, already having the form of the 
body with three surfaces: a, b, c â€” anterior, posterior, and 
upper surfaces; d â€” anterior angle; e, e â€” posterior angles; 
f â€” lower angle ; g â€” anus . 

Câ€” â€” cross-section of this same stage of development, in which 
for elucidation of the passage the previous apple-shaped 
form was drawn: a, b, c, d, e, f, g â€” as in figure B; 
h â€” rudiment of the digestive tract in immature form; 
k â€” its rudiment in later form. 

D â€” larva of the following form, back view: a, b â€” posterior and 
upper surfaces; c â€” anus (back passage) ; gg, 11, kk â€” rudiments 
of club-shaped, arch-shaped, and transverse branches of 
calcareous skeleton. 

E â€” later larva: a, b : c, g, k, 1 â€” as in figure D; dd â€” rudiment 
of posterior limbs; hh â€” first traces of hydrants in it. 

F â€” larva of pear-shaped form, back view: dd â€” posterior limbs; 
ee â€” anterior limbs; fâ€” -mouth; q â€” -anus; ssss â€” cilia apparatus. 

G â€” calcareous skeleton of the formed larva, represented in 

an isolated view: gg â€” club-shaped hydrants; hh â€” hydrants of 
anterior limbs; kk â€” arch-shaped hydrants; 11 â€” transverse 
hydrants . 



From Krohn's description it is clear that he saw, for 
the first time, in the cavity of echinus division that 
accumulation of cells, which was called by the latest 
embryologists the primary mesenchyma. He correctly determined 
the fate of these cells in larval development. This discovery 
of Krohn's was so thoroughly forgotten that Mechnikov, who was 
well acquainted with the old embryological literature, 
attributed it to Selenka, whose work was published exactly 
thirty years later after Krohn's monograph. 65 

"The development of Echinus micro tub eraulatus ," Mechnicov 
wrote, "was studied by Selenka. The first differentiation of 
the embryonic layers begins with the bulging of the lower 
cells of the blastoderm. Until the beginning of their 
protrusion they form a number of wandering cells, which later 
develop into the cutis and are considered mesodermal or 
mesenchymal cells. "66 

Krohn observed that after nearly thirty hours after 
fertilization the following important changes take place in 
the larva of Echinus lividus . In the center of the blunt pole 
a small hole-like deepening appears. The blunt pole widens and 
thickens; the hole in its center becomes deeper and wider, 
so that the body of the larva becomes similar in form of an 
apple. The more enlarged hole "formed as a result of that," 
Krohn wrote , caused the skin in this area gradually to protrude 
(EINSACKT ODER EINWARTSSTULPT) into the cavity 
of the body. The protrusion (EINSACKUNG) submerged 
deeper in the body cavity and extended into a canal 
which finally reached the walls of the body cavity 
toward the blunt pole. The sac which appeared by this 
means stretched through the body cavity (Figure 45, 
C, h, k3 and is the rudiment of the digestive cavity. 
The edge or circumference of the primary hole becomes 
a leading opening in the canal, which is the anus. In this view's favor â€” on the manner of appearance of 
the digestive tract â€” is the fact that it also 
indicates that the wall of the canal is absolutely 
equivalent to the skin in thickness and structure. 
(p. 18) 



65. E. Selenka, "Keimblatter und Organanlage bei Echinicen," 
ZTS. WISS. ZOOL.,, 33 (1879) . 

66. I. I. Mechnikov, "Vergleichend-embryologische Studien. 

3: Uber die Gastrula einiger Metazoen," ZTS. WISS. ZOOL., 
37 (1882) , pp. 286 - 313. 





Krohn's discovery of the nature of the formation of the 
larval intestine of echinus undoubtedly possesses outstanding 
significance. This, unquestionably, is the first description 
of the invaginated gastrulation phenomenon. Its study subsequently played a great role in the progress of comparative 
embryology. Comparative analysis of methods of separation of 
the endoderm is included in the basis of Kovalevsky f s evolutionary concept about the formation of the embryonic layers, 
and also in the basis of Haeckel's gastrula theory. Krohn's 
priority in this question was not definitively underlined. 
Mechnikov, in the article just cited, wrote the following. 

Derbes (1847) described the formation of the larva of 
Echinus esoulentus and mentioned a stage in the form of 
a double sac with skin layer in which the caecum opened 
to the outside. Twenty-five years later, Haeckel gave 
this the name gastrula, which was accepted by scientists 
all over the world. Derbes thought that the opening 
of the rudiment of the intestine was the mouth, but 
August Krohn (1849) showed that it corresponded to the 
anus of the pluteus. He described the process of 
protrusion itself. 

Krohn referred to Derbes' observations: "The reader can form 
an excellent concept of the gradual formation of the digestive 
tract just described by looking at Figures 13 and 14 in 
Derbes' article; however, the author apparently did not pay 
attention to the process of formation itself" (p. 19). The 
following is written by Derbes: "The spherical form of the 
larva is changed by pressure at one point of the surface. 
Gradually this pressure becomes more pronounced, and its 
center is penetrated by an opening which leads to a rudiment 
of the intestinal cavity. Beginning from this moment, the 
movement of this opening is always directed forward and, later, 
upwards. . . that is, the mouth looks towards the zenith" 
(pp. 91 - 92). It is clear that Derbes did not put the deepening 
on the surface of the spherical larva in genetic relation with 
the formed intestine. According to his opinion, the opening 
formed in the center of the deepening united in an unknown way the developed rudiment of the digestive tract with the 
external world. Krohn pointed also to DerbSs' mistaken assumption that this opening was the mouth. He established that the 
opening in the area of protrusion is the anal opening, and 
the mouth is formed in another place significantly later 
(on the fourth day after fertilization] . 

Further observations of Krohn are concerned with the 
changes in the larval form" as it becomes bilaterally symmetrical (Figure 45, B and C) . He described the formation of 
limbs and the calciferous skeleton (Tigure 45, D and E) , 
which gradually acquires a different configuration. 

In the following year, Krohn repeatedly returned to the 
study of development of the different echinoderms and 
published many reports on this subject, One of these reports 
considered the development of the holothurian and echinus. 67 
The larvae of holothurians (Holothuria tubulosa} were 
obtained from plankton, because attempts at artificial 
insemination proved unsuccessful. The youngest larva observed 
by Krohn "is similar to an elongated egg (Figure 46, A); in 
its sharp pole there is an opening leading to a sac-like 
protrusion in the body and an ampul la- shaped canal widening at 
the end Ik This sac is the rudiment of the digestive tract, 
the opening doubtlessly is the anus" (p. 345). Krohn noticed 
that the larva of Holothuria tubulosa, at this stage, is very 
similar to the larva of Echinus lividus described earlier by 
him (1849) . Similar to the latter, the larva of Holothuria 
tubulosa is covered by cilia, and with their help it swims with 
the imperforate pole forward. The surface of the body and the 
digestive tract consist of cells which, with their nuclei, 
become noticeable with the addition of fresh water. In the 
body cavity, as in the larvae of Echinus lividus, fibrous 
tissue with fusiform cells is found. Later (Figure 46, B) on 
the abdominal side of the larva a depression appears â€” which 
is the future transverse fissure of ausicularia; in this 
fissure the mouth opening later appears. Later the body 
acquires a kidney-shaped configuration (Figure 46, C) . The 
digestive tract, at this time, forms the rudiments of the three 
parts, pharynx e, stomach d and intestines c. The appearance 
of the mouth opening coincides with the beginning formation 
of the ciliary strings in the auricularia. At this stage there 
is some data about the transformation of the Echinus lividus . 



67. A. Krohn, "Beobachtungen aus der Entwickelungsgeschichte 
der Holothurien und Seeigel," ARCH. ANAT . , PHYSIOL. 
(1851) , pp. 344 - 352. 



Krohn's other reports on the development of echinoderms 
briefly designate some representatives of the type68 or 
species. 69 

Special attention must be given to Krohn's investigations on the development of tunicates. Originally his interest 
was attracted by Salpa, in which the wonderful phenomenon of 
the alternation of sexual and asexual generations had already 
been recognized. This discovery belongs to the poet Adalbert 
Chamisso (1781 - 1838) . With the Dorpat-born zoologist 
J. F. Eschscholtz, he traveled around the world on the 
Russian ship RURICK. Chamisso published the results of his 
investigations in "On Some Animals of the Linnean Class of 
Worms, noted during a world tour, performed by Count 
N. Romanzoff, under the command of Otto von Kotzebue, from 
1815 to 1818, Part I: On Salpa. "70 



68. A. Krohn, "Bemerkungen uber einige Echinodermenlarven, " 
ARCH. ANAT., PHYSIOL. (1851), pp. 353 - 357; "Uber 
die Entwickelung der Seesterne und Holothurien (Brief 1. 
Mitt. a. d. Herausgeb . ) , " ibid . (1853), pp. 317 - 321; 
"Beobachtungen iiber Echinodermenlarven (Brief 1. Mitt, 
a. d. Herausgeb.)," ibid . (1854), pp. 208 - 213; "Uber 
neuen Entwickelungsmodus der Ophiuren," ibid . (1857), 
pp. 369 - 375. 

69. A Krohn, "Uber die Entwickelung einer lebendiggebarenden 
Ophiure (Brief 1. Mitt. a. d. Herausgeb.)," ARCH. ANAT. 
PHYSIOL. (1851), pp. 338 - 343; "Uber die Larve von 
Spatangus purpureus (Brief 1. Mitt. a. d. Herausgeb.)," 
ibid . (1853) , pp. 255 - 259; "Uber die Larve des Echinus 
brevispinosus ," ibid . (1853), pp. 361 - 364. 

70. DE ANIMALIBUS E CLASSE VERMIUM LINNEANA IN 
CIRCUMNAVIGATIONE TERRAE AUSPICANTE COMITE 
N. ROMANZOFF DUCE 0TT0NE DE KOTZEBUE ANNIS 
1815, 1816, 1817, 1818 PERACTA OBSERVATIS 
ADELBERTUS DE CHAMISSO, Fasc. primus: DE SALPA 
(Berlin, 1819) , iv + 24 pp. 



Figure 46. Three stages of the development of the sea 
cucumber Holothuria tubulosa (by Krohn) . 



A and B: a â€” anus; b â€” digestive cavity, 
Id â€” anus; c, d, e â€” rudiments of the 
:estine, stomach and pharynx. 



C: 
inte 



The alternation of sexual and asexual generations in 
Salpa was described by Chamisso as follows: 

The species of Salpa is found in double form: each 
generation of the species is dissimilar to its 
parents, but through birth posterity is similar to the 
last, so that any Salpa differs from its parent but 
is identical with its grandparents. Both forms are 
similar to headless molluscs, hermaphrodites or the 
female sex. Both of them are viviparous, but one of 
them is a solitary animal, the originator of many 
descendants . The other represents a complicated 
branch consisting of animals, each united with the 
others by the necessary connection which gives birth 
to one descendant. These changed forms of the unchanged 
species are called solitary (Proles solitaria) and 
aggregated or colonial (Proles gregata) generations, 
(p. 2) 


After the discovery by M. Sars^l of the analogical 
change of generations in scyphomedusa, Steenstrup united 
these facts to produce one biological regularity. Steenstrup 's general conclusion was highly regarded by his 
contemporaries, in particular by Baer.72 Krohn also gave 
great significance to Chamisso's discovery and Steenstrup 's 
ideas. He made it his task to study in detail the reproduction 
and development of Salpa. To do this, he settled for many 
months on the coast of Sicily, where the sea provided him the 
necessary material. Krohn put the results of his observations 
into a special work, 73 in which, first of all, he completely 
confirmed Chamisso's observations, by distributing them over 
seven species of Salpa which were, for the first time, partly 
described by him. The comparison of the solitary and colonial 
forms (Proles solitaria and Proles gregata, in Chamisso's 
terminology) allowed Krohn to regulate the taxonomy of this 
group of tunicates. He showed that salpae described under 
different names frequently proved to be different stages of 
the development of one and the same species (137) . Later, 
Krohn gave the characteristics of the structure of the 
heteromorphic generation, and also described the ovum, the 
seminal glands, and the process of fertilization in the 
sexual generation. 

Within the present book, the greatest significance is 
placed on the section in Krohn' s article (111) in which he 
discussed development of the embryo in the maternal organism. 
After fertilization, the embryonic vesicle and the embryonic 
spot disappear, after which the ovum enlarges in size and 
acquires a regular spherical form. This was according to Krohn, 
who was not completely convinced of its authenticity. Sometimes 
the ovum is not seen like that, and in its place a round body 
appears, raising a region of tunica of the mother and jutting 
into its cavity in the form of papilla. This body, Krohn wrote, 
is nothing other than the rudiment of the placenta which, by 
deepening in the cavity of the body of the maternal organism, 
enters in connection with two of its blood vessels. Only 
after the formation of the placenta does the embryo begin to 
develop, at first in the form of a very small body appearing on the summit of the placenta under its cover (138) . In this rudiment of the embryo all organs of the last are developed; 
"however," Krohn wrote, "all that concerned its development 
during the first period remained for me almost completely 
unknown" (p. 123). He could only establish that one of the 
first organs of the embryo by the time of appearance is "the 
respiratory cavity." The embryo changes from compact to hollow, 
following which the rudiments of the branchae and nervous 
ganglion are already seen, while the organs "visceral nucleus" 
and heart become noticeable only later. Only after that does 
the embryo acquire a definite form; the anterior and posterior 
openings appear in it. At the end the embryo becomes more 
voluminous than the placenta, and all its organs intensively 
enlarge, especially the nervous ganglion, from which numerous 
nervous branches grow. At the same time muscular strips and 
blood vessels appear, which are not completely formed. 




71. M. Sars, "Uber die Entwickelung der Medusa aurita und Cyanea 
caprtlata," ARCH. NATURG . , 7 (1841), pp. 9 - 34. 

72. See Chapter 23. 

73. A. Krohn, "Observations sur la generation et le developperaent des Biphores (Salpa)," ANN. SC . NAT., 3S6r., 
Zool., 6 (1846), pp. 110 - 131. 




Krohn' s concluding paragraph of the work is dedicated 
to the processes of budding in the asexual regeneration of 
Salpa, and to the formation of colonies; the character of the 
last varies in different species. Here the description of the 
stolon and the embryos is given, situated along it so that 
their axes cross the stolon at a right angle. These embryos 
develop in definite succession, depending on their situation 
on the stolon. 

The development of salpa, especially the formation of 
their embryos from fertilized ova, represents one of the most 
difficult principles of embryology. Krohn' s investigations 
began this study and recent opinions are credited to many 
Russian embryologists at the end of the nineteenth and the 
beginning of the twentieth century, including A. 0. Kovalevsky, 
M. M. Davydov, A. A. Korotnev, and V. V. Zalensky. 74 

Six years after the publication of the above-mentioned 
work, Krohn published an article presenting the results of 
his investigations of the little-studied group of tunicates, 
the doliolum.75 Q U oy and Gaimard made a voyage on the 

74. "The first investigations of the development of Salpa go 
back to. Krohn," K. N. Davydov wrote in his handbook 
CTRAITE, p. 867) . 

75. A. Krohn, "Uber die Gattung Doliolum und ihre Arten," 
ARCHIV. NATURG., 18 (1852), pp. 53 - 65. 


ASTROLABE 7 ^ to Road Island, Ambon (in the Moluccas), and 
to the coast of the Vanikoro Islands (between the New Hebrides 
and the Solomon Islands) . During this time they discovered 
this small transparent tunicate, which they described and 
presented only incompletely, and for which they suggested the 
generic name Doliolwn. 

Later T. Huxley, on a tour around the world aboard the 
ship RATTLESNAKE, saw doliola in the southern part of 
the Pacific Ocean and described their structure more exactly 
and in more detail than Quoy and Gaimard,77 but in his notes 
the male genital glands were given for individuals of sexual 
regeneration. 

Krohn, for the first time, discovered the presence of 
doliola in the Mediterranean Sea (near Messina and Naples) , 
studied the structure and described three new species. 
According to Krohn, doliola "are free living ascidians, but 
in many respects are similar to salpa and form, therefore, an 
interesting intermediate link between both these orders of 
tunicates" (p. 53). From Krohn's description, the structure and 
reproduction of doliolum, according to the majority of the 
features, must be related to salpa; this applies also in the 
present taxonomy. 

Krohn observed that from the ova of doliola cercarialike larvae develop which later undergo metamorphosis . 
The peculiarities of the larvae are connected, according 
to Krohn, with the mode of life of the tunicates more than to 
ascidians, which are in their adult condition fastened motionless to the substrate. In accordance with this, in the ascidian 
larvae the tail disappears early, but in the larvae of doliola 
it remains throughout metamorphosis and serves as an organ 
of movement. The process of reduction of the tail in the larvae of doliola takes place as in the larvae of ascidians . In 
the latter the disappearance of the tail was noticed for the 
first time by Milne-Edwards in an example of the colonial 
ascidian Amouruoium proliferum, and was described in more 
detail by Krohn for Phallus-La mammillata . 



76. VOYAGE DE DECOUVERTE DE L' ASTROLABE EXECUTE PAR 
ORDRE DU ROI PENDANT LES ANNEES 1826 - 29 SOUS 
LE COMMANDEMENT DE M. J. DUMONT D'URVILLE. 
ZOOLOGIE PAR MM QUOY ET GAIMARD, vol. 3 (1834). 

77. T. Huxley, "Remarks upon Appendicularia and Doliolum, 
Two Genera of Tunicates," PHIL. TRANS. ROY. SOC . 
LONDON (1851), pp. 599 - 602. 





The metamorphosis of doliola was again studied by Krohn 
on the species Doliolum Nordmanni. Krohn represented two 
stages of transformation; Figure 47, A represents the stage 
when the tail of the larva has not begun reduction; and 
Figure 47, B, the larva with shortened tail. Until metamorphosis the tail of the larva is tapered, with the two ends 
covered with gelatinous membrane a_; its axis d_ is composed 
of cuboidal cells situated in one row, so that it seems to 
be segmented. The root of this axial shaft penetrates into 
"the vesicular appendix" c_, lying under the digestive tract 
of the larva. This vesicle decreased simultaneously with the 
reduction of the tail and at the end it disappears (139) . In 
the stage represented in Figure 47, B, all organs in the 
larva are already differentiated; on its dorsal side there is 
the placing of the stolon Â£. 

Krohn ! s description of the structure of the larva of 
doliolum and its transformation distinctly shows that he saw 
in it all the principal features of organization, on the basis 
of which the doliolum was later counted as a type of chordate 
(subtype tunicate, class salpa) : the presence of the reduced 
cord at the time of metamorphosis ("axis of tail"), the situation of the nervous ganglion o_ on the dorsal side and the 
heart m on the ventral side. 

Correct evaluation of Krohn ! s investigations was given 
by V. N. Ulianin in the classic monograph on doliola, first 
published in Russian, and two years later in German.' 7 ** Ulianin 
wrote the following: "Soon after the appearance of Huxley's 
article the doliolum was found in the Mediterranean by Krohn. 



78. V. Ulianin, ON THE DEVELOPMENT AND REPRODUCTION OF 
DOLIOLUM (Moscow, 1882), 100 pp. B. Uljanin, DIE ARTEN 
DER GATTUNG DOLIOLUM IM GOLFE VON NEAPEL UND DEN 
ANGRENZENDEN MEERABSCHNITTEN, FAUNA UND FLORA 
D. GOLFES V. NEAPEL, X. Monographie (1884), 140 pp. 


The article in which this discovery was published possesses 
important significance in the history of doliolum, as it 
contains the first observations on the development of this 
animal. Krohn not only described the tailed larva of doliolum, 
tut he also pointed out the alternation of generations in the 
reproduction of this animal.... On the basis of his observations Krohn concluded that from the ovum of doliolum is formed 
the tailed, freely swimming larva, which after the 
loss of the tail is transformed into an asexual doliolum" 
(p. 2). Later on Ulianin noticed Krohn's mistake in dividing 
doliolum according to the number of muscular strips. In 
all species of sexual generation there are eight, but in 
asexual species there are nine strips. Therefore the species 
described by Krohn, Doliolum Trosoheli , is in fact an asexual 
individual D. dentioulatum Q. and G., and D. Nordmannz Krohn 
is the asexual generation of D. Mulleri Krohn. Particular 
significance was given by Ulianin to Krohn's embryological 
observations. "All that is known presently about the 
embryological development of Doliolum," Ulianin wrote, 
"comes exclusively from Krohn, who for the first time described 
the free larva. All later authors.... only redescribed it, not 
adding anything essential to Krohn's description (p. 47). 

The central place among Krohn's investigations of the 
development of tunicates is occupied by his work concerning 
the solitary ascidians.? 9 The artificial insemination in 
ascidians which was used successfully for the first time by 
Baer, Krohn also used, observing the development of Phallusia 
mammillata step by step for three months. He described the 
mature ovum of this ascidian in the following way. The ovum 
present in the ovum-fluid is supplied by papillae and covered 
by a cover membrane under which the proper ovum membrane is 
present. Somewhat deeper lies a hyaline membrane containing 
inclusions which is green in color. The yolk itself is 
colorless, the embryonic vesicle and the embryonic spot in 
the mature ova are unnoticeable . The above mentioned green 
hyaline membrane was considered by Krohn, following MilneEdwards, a source of formation of tunica. The error of this 
view was later established, but it was repeated in many 



79. A. Krohn, "fiber die Entwickelung der Ascidien," ARCH. 

ANAT., PHYSIOL. (1852), pp. 312 - 333. In the following 
year this work was published in English as "On the 
Development of the Ascidians," SC. MEM. NAT. HIST. 
(1853) , pp. 312 - 329. 


subsequent embryological works . The division of the ovum 
begins two to three hours after contact with the sperm. 
Krohn considered that the division, at least in the first 
stages, follows the rule of progress. The vesicular nuclei 
of the globules of division disappear before every division 
and then again become visible. "Instead of nuclei," Krohn 
wrote, 

in every divided globule an absolutely peculiar 
distribution of yolk molecules is noticed. Namely, 
they are distributed in the form of strands, which are 
directed from the depth, from the medial point by 
radius in all directions to the lighter periphery of 
the ovum and, apparently, come out from two centers of 
irradiation. After the end of division, inside the 
new globules the nuclei again become noticeable, then 
these radiant figures disappear and the yolk granules 
are found to be situated close to each other, (p. 315) 

These observations show that Krohn exactly described 
many details of mitotic division in blastomeres. 

Embryonic development goes quickly, and a day after 
fertilization a cercaria-like embryo with a more or less 
developed tail is already present in the ovum membrane. The 
body and the tail of the embryo are composed of cells which 
are especially noticeable on the surface. The cells have a 
polygonal form, and contain granules and nuclei in the center. 
The axis of the tail, according to Krohn 1 s description, is 
composed of larger rectangular cells with nuclei situated in 
a row, one following the other, and therefore they have a 
striated or a disjointed form (Figure 48, A, b) . 

Shortly before the final formation of the larva the tail 
undergoes remarkable transformations. According to Krohn' s 
observations, they amount to the following. The axis is 
transformed into a canal, as its cellular structure gradually 
disappears due to the destruction of the partitions between 
the neighboring cells and the liquefaction of their contents. 
The small cells surrounding the central tail strand are 
transformed into longitudinal muscular fibers. On the dorsal 
side of the larva at first appears one, and then behind it 
another, pigmented spot of granular origin (Figure 48, A, d, e 
and B, e, f ) . At the time of transformation this formation is destroyed, and the pigment passes into the blood channel. 
The formed larva is set free from the membrane by the tail 
movement. The body of the larva CFigure 48, B) in the anterior 
end is supplied by three similar processes on the sucker. The 
larva is soon attached at the anterior end and undergoes 
transformation, one of its marks being the disappearance 
of the tail. Milne-Edwards saw only that the axial part of 
the tail is set free from its covered sheath and extends into 
the body of the larva, but he did not elucidate the subsequent 
fate of this formation. "By my observations," Krohn noted, 

the setting free and the extension of the tail axis , 
the deep immersion by the tail in the body of the larva 
only precedes the processes of reduction which it soon 
undergoes. Directly after the extension, the tail axis 
remains undamaged at the posterior part of the body. 
It is situated here convoluted into a spiral coil . . . . 
With the beginning of the development of the young 
ascidian, this coil first disintegrates into a large 
number of strips situated close to each other, which 
then are gradually destroyed; the number and size of 
the strips decrease, but the insignificant remnant 
does not disappear entirely. (Figure 48, C) (pp. 318 319) 

Krohn himself considered his observations on the development of ascidians incomplete, and he acknowledged only the most 
essential changes. He described in particular the formation of 
the vessels of the tunica and the development of the respiratory cavity, or gill cleft, and behind it the rudiment of the 
digestive canal in the form of a loop-shaped canal . Somewhat 
later three openings on the spinal side of the body appear: the 
most anterior, the inlet into the respiratory cavity and 
digestive canal, and two posterior which later merge together 
in a common excretory opening. Simultaneously, the nerve 
ganglia develop in an elongated formation in the middle of the 
back near both pigmentation spots. Near the nerve ganglia the 
rudiment of the muscular strands form and the dorsal fissure 
appears. The digestive canal is differentiated into three 
parts: a canal which opens into the respiratory cavity, stomach, 
and intestine. In the walls of the respiratory cavity there 
develop near the stomach the first branchial clefts with cilia 
at the edges, and at the ventral fissure the heart develops, 
possessing the form of a short duct. The metamorphosis is 
completed by the specialization of the gill-clefts and the 
development of siphons . 



Figure 47. Larvae of Doliolum Nordmanni . 

A â€” Larva up to transformation: a â€” larval 
membrane; bâ€” young doliolum; e, f â€” posterior 
and anterior opening; d â€” axis of the tail; 
B â€” larva of doliolum in the beginning of 
transformation: a, d, e, f â€” as in A; 
c â€” "vesicular appendix; gâ€” wall of the 
respiratory cavity; h â€” -digestive tract; 
k â€” stomach; 1 â€” intestine; m â€” heart; nâ€” ventral 
fissure; o â€” nervous ganglion with outgoing nerves; 
pâ€”â€” the third from back muscular strand, penetrating the rudiment of the stolon (q) (by Krohn) . 


Krohn's work represents the first systematic description 
of ascidian development in world literature; it remains incomplete and not free from mistakes, which, of course, does not 
reduce the historical significance of this undoubtedlyremarkable investigation. But the exact and detailed study 
of the embryology of ascidians belongs to A. 0. Kovalevsky. 
In his work, their relationship to the vertebrates was proved, 
delivering a fatal blow to the metaphysical theory of types 
in the animal kingdom. It formed the basis of comparative 
evolutionary embryology, first advanced by Krohn. In the work 
dedicated to the development of ascidians, 80 Kovalevsky wrote 
the following: 

Leaving aside the investigations of earlier authors, 
whose results either are already completely reworked 
by present scientists, or, to a lesser extent, are 
partially expanded, we must mention Milne-Edwards, 
Van Beneden, Kolliker, and, in particular, Krohn. Of 
all these investigations the results of Krohn's 
investigations are in closest agreement with our own 
results. Although he described the accumulation of 
pigments, which completely coincides with our observations , he did not discover the walls of the saccule in 
which these organs of sensation are situated, and 
generally he traced the development step by step. The 
formation of the axial strand in the tail of the 
ascidian larva was observed by Krohn, although he 
explained it as a formation of emptiness in the cells . 
Although the transformation of the larva into the 
sessile form was described by him in detail, he had 
only a slight understanding of the anatomy of the larva 
and therefore he could not observe the particular 
features, (p. 41) 




80. A. O. Kovalevsky, "Istoriya razvitiga prostykh astsidii" 

(The history of development of the simple ascidia) (1886) , 
SELECTED WORKS (Izd. AN SSSR, 1951) , pp. 41 - 78. 

81. A. Krohn, "Uber die fruheste Bildung der Botryllusstocke," 
ARCH. NATURG., 35 (1869), pp. 190 - 196; Uber die 
Fortpflanzungsverhaltnisse bei den Botrylliden, " ibid ., 
pp. 326 - 333. 


At the end of the 1860s, in Naples, Krohn studied budding 
in the complex ascidian Botryllus , and he presented the results 
of his observations in two reports. 81 In the first of these articles Krohn disproved the erroneous data of Milne-Edwards 
and Sars and confirmed the observations of Mechnikov that the 
larva of Botryllus possesses the same simple structure as 
that of the solitary ascidian and undergoes analogical 
metamorphosis. After settling on the bottom, the young 
Botryllus , already in the process of transformation, produces 
a bud from which a second individual originates, which in 
turn begins to bud. As a result a stellate colony is obtained. 
The budding of the colonial ascidian is represented in the 
second article, in which Krohn compared its details with the 
corresponding phenomena in salpa. 

With these fragmentary investigations of vegetative 
multiplication, Krohn' s scientific activity apparently came 
to an end. In the following years (1870 to 1880) his reports 
were regularly placed in journals, but no works appeared after 
this time. 

For thirty-five years he collected facts from the field 
of anatomy and embryology, mainly of invertebrates, covering a 
very great number of systematic groups CI 40) . His embryological 
works (including descriptions of larvae and means of reproduction) concerned coelenterates, nemerteans, annelids, molluscs, 
crustaceans, echinoderms, and tunicates. 

Krohn did not belong to those investigators paving new 
roads in science. All his comparative-anatomical and comparativeerabryological remarks are concerned with the comparison of 
closely related forms. Comparing, for example, representatives 
of different classes of echinoderms and establishing the 
features of similarity and difference between salpa and 
ascidians, Krohn did not offer sympathy either to the theory 
of types, nor to the idea of unity of planes, nor to evolutionary study. Krohn cited Darwin only in his works on the 
structure and development of cirripedes, highly rating his 
monograph dedicated to this order of crustacean. 

Besides this, evidence exists about Krohn' s deep interest 
in the investigations of A. 0. Kovalevsky, who established the 
similarity of the embryonic development of ascidians and 
vertebrates. The first report by Kovalevsky dedicated to the 
development of ascidians was published in 1866 in ZAPISKAKH 
PETERBURGSKOI AKADEMII NAUK (Notes of the Petersburg 


Figure 48. The development of the ascidian Phallusia 
mammillata (by Krohn) . 


A â€” late embryo with two pigmentation spots: a â€” body; 

b â€” beginning of the tail; c â€” rudiment of upper sucking 
processes? d, e â€” anterior and posterior pigmentation 
spots . 

B â€” larva, side view: aâ€” tunica with green bodies; bbâ€” the 
axis of the tail; cc â€” its canal; d â€” horizontal half; 
e, f â€” anterior and posterior pigmentation spots; 
g â€” right anterior; h â€” posterior sucking processes. 

C â€” Phallusia in the process of metamorphosis: aâ€” widely 
opened respiratory siphon; bbâ€” posterior (constrictor) 
siphons; c â€” nervous ganglion with the nervous branches; 
d: â€” digestive tract; e â€” stomach; f â€” intestine; g â€” situating 
coil of the larval tail; hhhh â€” first two pairs of gill 
openings of the respiratory sac; i â€” pigmentation mass over 
the nervous ganglion; k â€” ventral fissure; 1, 1 â€” tunica. 



Academy of Science); two years later an article followed in 
NACHRICHTEN VON DER GESELLSCHAFT ZU GOTTINGEN. The 
data mentioned there, apparently, did not convince Krohn. In 
1871 in ARCHIV FUR MIKROSKOPISCHE ANATOMIE, edited by 
Max Schultze, a new work by Kovalevsky was published, 82 a t 
which time Schultze added the following postscript to a letter 
directed to Kovalevsky on January 18,1871: "Krohn, who read 
your article in proof and made minor corrections, sends you 
heartfelt regards. At first he was, as you can imagine, very 
much against the relationship with vertebrates; 83 b u t then he 
began to hesitate." 84 

Krohn was one of the pioneers of zoological investigations on the Mediterranean coast, which later became a place 
of pilgrimage for naturalists from different countries of 
Europe. 

It is highly probable that Kovalevsky and Mechnikov were 
well acquainted with the works of Krohn. His remarkable 
comparative embryological investigations, of which they became 
aware in the mid-1860s, revealed the nature of Krohn 1 s 
scientific activity and produced an impression on them. Like 
Krohn, they spent many years of their lives as travelling 
naturalists, more than once following him to those localities 



82. A. O. Kovalevsky, "Entwickelungsgeschichte der einfachen 
Ascidien," MEM. AC. SC . ST. PETERSB . , VII Ser., 10, 
No. 15 (1866), 16 pp.? "Beitrag zur Entwickelungsgeschichte 
der Tunicaten," NACHRICHTEN VON DER GESELLSCHAFT 
ZU GOTTINGEN, No. 19 (1868), pp. 401 - 415? "weitere 
Studien iiber die Entwickelung der einfachen Ascidien," 
ARCH. MIKR. ANAT., 7 (1871), pp. 101 - 130. 

The Russian translation of the first and third articles 
are to be found in A. O. Kovalevsky, SELECTED WORKS, 
editor and commentator A. D. Nekrasov and N. M. Artemov 
(Izd. AN SSSR, 1951), pp. 41 - 78 and 79 - 122. 

83. The paper was on the relationship of ascidians to vertebrates . 

84. Schultze' s letter is included in the book PEREPISKA 
A. 0. I V. 0. KOVALEVSKY (Postscripts of A. O. and 
V. O. Kovalevsky) , edited by A. A. Borisiak and 

S. Ya. Streich. The extract of this letter is published 
here with the permission of S. Ya. Streich. 


rich in the zoological material of the sea â€” Naples, Messina, 
Nice, Madeira â€” and using, in particular, those subjects on 
which Krohn made many important observations leading to 
serious theoretical meditation. Kovalevsky later cited with 
respect the works of Krohn on the structure of sagitta and 
the development of tunicates, and Mechnikov cited his works 
on the development of coelenterates and echinoderms . 

In the preparation of that revolution in embryology 
which was accomplished by Kovalevsky and Mechnikov, converting 
the comparative-descriptive embryology into comparative 
evolutionary embryology, Krohn 's modest investigations played 
their role, and therefore his name must not be forgotten in 
the history of Russian science.



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