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=Experimental Studies On The Origin Of Monsters=

==II. Regarding The Morphogenesis Of Duplicities==

E. I. Werber 

From the Osborn Zoological Laboratory, Yale University 

Twenty-Seven Figures 

* This contribution is a part of the work carried on with the aid of a grant from the Bache Fund of the National Academy of Sciences in 1915. 

==Introduction==

In a recent publication (Werber, '16 b) I have attempted 
an etiology of monstrous development and an analysis of the 
morphogenetic factors underlying it. The observations there 
recorded and the theoretical conclusions based on them pertained largely to terata of the head, but some other experimentally produced forms of pathological development have also 
been sufficiently considered.* 

The present paper is a continuation of this work. Its primary purpose is to put on record some duplicities which have 
resulted from the employment of a chemical method. The 
attempt is also made to account for their morphogenesis although I am fully aware of certain possible inadequacies of our interpretation which may be largely due to the insufficiency of 
data at command at the present time. 


The term 'duplicity' has, unfortunately, lost its original meaning and come 
to be employed almost exclusively in its metaphoric sense. In view of the lack 
of an other suitable English term I propose henceforth in dealing with monozygotic double embryos to employ it in its original meaning which I believe to be 
proper and precise, being the equivalent of the Latin 'duplicitas' and the German 'Doppelbildung.' The commonly employed term 'double monsters' ('Doppelmissbildung') can be employed only for deformed conjoined double embryos. 



3 In a very recent paper Newman (Biological Bulletin, vol 32, No. 5, 1917) 
makes the statement that I failed to account for such teratomata as the 'isolated 
eye' (or the 'solitary eye'). In reply I refer the reader to pp. 541-552 of my 1916 b 
paper where these teratomata, for the first time experimentally produced by 
myself, are described and their morphogenesis fully accounted for. 



I also appreciate the justification of an objection often raised 
against such attempts, namely that deductions regarding the 
morphogenesis of monsters are uncertain, if based only on the 
analysis of the end-products of such atypical development. 4 It 
is, of course, obvious that certainty could here be gained only 
by direct observation in vivo from the beginning of development 
through successive stages. However, this direct method presents great difficulties in view of which it seems necessary to 
take into account the inferential clues often furnished by the 
analysis of already developed monsters. 5 The indirect evidence 
thus gained is in accord with data established by direct observation of successive stages during atypical development in 
many experiments in the lower animals. 

==Blastolysis as a Morphogenetic Factor in the Development of Double Monsters==

"While attempting to account for the morphogenesis of 'monstra per defectum' I (I.e.) have shown that in many of the embryos of which a microscopical study was made there was unquestionable evidence of either dissociation of tissues or of 
dislocation of organs or of both. From these observations 6 
the conclusion was drawn that we are here, manifestly, confronted with some action (or actions) that tended to disintegrate 
and to dissociate parts of the earliest embryonic primordium. 
As the chief components of this complex process (blastolysis) 

'' 4 These objections were pointed out already by Rauber ('79-'80). 

5 Every teratological experiment in which, like the present, a chemical modification of the environment is employed, must be carried out on a great many 
eggs, since their mortality is very high. This greatly increases the difficulty of 
•continuous observation of the still surviving eggs of which probably no two develop alike. Besides, the chemical processes and the alterations they are responsible for, elude direct observation. This is particularly true for the translucent egg of Fundulus. 

6 Similar observations were made by Mall ('08) who, however, dealing with 
material not coming from experiments, did not attempt to account for the factors 
underlying this shifting and dissociation of tissues which he correctly pronounced as 'histolysis.' 


I regard destruction of parts of the primordium by chemical 
action and fragmentation resulting from the latter as well as 
from differences between the osmotic pressure in the eggs and 
that of their surrounding, experimentally modified, medium. 

These conclusions can, I think, be extended to the morphogenesis of 'monstra per excessum.' For Driesch ('93) has 
demonstrated for the sea-urchin and Wilson ('93) for Branchiostoma that the development of various 'monozygotic' 7 
double or multiple embryos can be induced by the employment 
during the first or the second cleavage of a dissociating force 
such as raising the temperature or shaking the eggs, which 
would separate the blastomeres. Similar results were obtained 
also by Fischel ('98) by mechanical pressure exerted on the eggs 
of the ctenophore Beroe ovata. It was further shown by Loeb 
('95) that like results can be obtained in sea-urchin eggs transferred soon after fertilization to a medium of lowered osmotic 
pressure. 

Of particular interest for our consideration is also the work of 
Bataillon ('01) who produced duplicities experimentally in lower 
vertebrates (Petromyzon and the teleost Leuciscus rutilus) by 
increasing the osmotic pressure of the surrounding medium. 
Employing this method Bataillon was able to ascertain by direct 
observation in Petromyzon that the development of monozygotic 
twins and various other duplicities resulted from the more or less 
complete separation of the blastomeres of the two-celled stage. 
He also states that in Petromyzon at least one-third of the egg 
seems to be necessary for the development of a whole embryo. 

This equi- and totipotency of the first blastomeres was demonstrated also in the amphibians. Thus O. Schultze ('94) and 
Wetzel ('95 and '96) have succeeded in producing conjoined double 
embryos in Rana fusca by inverting for some length of time 
eggs in the two-celled stage, previously compressed between two 
glass plates. 

Even complete monozygotic twins (of half the normal size 
may be obtained in amphibians by separating the blastomeres, 
as shown by the ingenious experiments of Herlitzka ('95 and '97) on the eggs of Triton cristatus. These interesting results 
were subsequently supplemented by the important experiments 
of Spemann ('00-04) who produced various duplicities in Triton 
taeniatus by incomplete constriction of the eggs along the first 
cleavage plane or along the corresponding plane at any stage 
up to the gastrula. 

7 This very precise term is adopted from Newman ('17). 





Bearing in mind these data as well as also Morgan's much 
earlier ('93) discovery of the totipotency of the blastomeres 
after the first cleavage in the teleost egg (Fundulus) it would 
seem perfectly safe to assume that the duplicities recorded in 
our experimeuts (on the eggs of the same species) have resulted 
from (primarily osmotic) dissociation ('blastotomy' — Bataillon, 

I.e.)- 8 

The origin of the double embryonic anlage can, in this way, 
be accounted for without difficulty — at least in anamniotes. 
So far as this point alone is concerned it would seem no longer 
necessary to resort to such assumptions as ' dichotomous growth' 
at the anterior end of the embryonic anlage (Gerlach, '82), 
or the oft-assumed binucleate eggs and coincidental polyspermy 
as the agent causing duplication, or the 'radiation'-theory 
of Rauber ('79-80) and its modified offshoot, the 'double gastrulation'-theory of 0. Hertwig ('92, '06). 

The inadequacies of all these hypotheses have been clearly 
pointed out by Fischel ('02) and in full agreement with the latter also by Schwalbe ('07). To Fischel we also owe what may 
be regarded as the most rational analysis of the manner in which 
the various combinations of duplicities in teleosts may be formed. 

Fischel's considerations are based primarily on the assumption of a duplication of the germinal anlage soon after fertilization probably by abnormal osmotic pressure, as demonstrated 
by Bataillon (I.e.). Starting thus with two embryonic primordia 
and bearing in mind the mode of formation of the embryo as 
established (particularly for the teleosts) by several authors 
and notably by Kopsch ('96) Fischel, by employing the dia
8 This assumption might reasonably be extended to the double embryos often 
recorded in fish hatcheries (notably in Salmonidae), where an abnormal osmotic 
pressure may be due to temporary impurities of the water. 



grams of the latter demonstrates with almost mathematical 
precision how the development of any diplopagus may result 
from the variation in the degree of approximation and the 
angle of incline of two embryonic primordia. 9 It is not practicable at this place adequately to present Fischel's views. They 
have, however, been fully discussed by Schwalbe (I.e.) to whom 
as well as to Fischel's paper the reader must be referred. Here 
it may suffice to say that his analysis accounts successfully for 
the morphogenesis of all double monsters, some of which (like 
the diprosopos, the dicephalos, the catadidymus and the mesocatadidymus) have eluded all other attempts at interpretation. 

However, it must be admitted that absolute proof for all of 
Fischel's postulates is wanting for vertebrates higher than 
Petromyzon, since Bataillon. unfortunately, made no observations regarding the morphogenesis of the double embryos he 
produced by osmotic pressure in Leuciscus. Nor is such proof 
furnished by the experiments on amphibians (referred to above), 
as they demonstrate only how certain duplicities may result 
from the separation of equipotent parts of the egg, but give us 
no clue regarding the formation of various other duplicities (in 
other vertebrates) by secondary fusion of duplicated primordia, 
as assumed by Fischel. 

It may also be mentioned, in passing, that Fischel's theory 
(blastotomy by osmotic pressure and subsequent chance configuration of the duplicated primordia) does not account for the 
malformations often exhibited by duplicities. The point is 
not an unimportant one, for as I shall attempt to show, these 
deformities may often be syngenetic with the doubling of the 
primordium. Besides, as will be pointed out later, the deformities of one or both components of a duplicity give us an important clue to the morphogenesis of the 'parasitic double 
monsters.' 

9 The various possib.le combinations in man were portrayed very suggestively 
in the well known diagrams of Wilder ('04) who, however, in his more recent 
worjv ('08) has abandoned his former views on diplogenesis in favor of his present 'theory of Cosmobia.' 


The deformities observed in duplicities are of the same nature 
as those of other monsters, i.e., they are, usually, due to some 
defects and, sometimes, 10 to an inhibition in development. 

It is relatively easy to account for the genesis of defects as 
the result of lesions produced by osmotic pressure. 11 The 
latter, however, will not account for an inhibition of development such as may be found in some monsters and, occasionally, 
in one or both components of a duplicity. As I have pointed 
out elsewhere (Werber, '15 and 16 b) for this form of deviation 
from the ontogenetic norm we must assume chemical alteration as responsible for the decrease of the germ's inherent capacity for development and differentiation. This assumption 
will account for such deformities observed in duplicities as may 
properly be regarded as due to an inhibition. 12 

Moreover, chemical action may play a much greater part in 
contributing to the genesis of defects by either directly destroying certain parts of the germ or by lowering their resistance to 
the action of osmotic pressure. The combined action of these 
factors (osmotic pressure and chemical alteration) may result in 
any conceivable monstrosity depending entirely upon its degree 
and upon the part of the germ which has suffered most from it. 13 

Keeping this in mind we shall be able to understand how not 
only (variously malformed) duplicities may result from the separation of blastomeres, but also how such monsters may 
come about through dissociation at a later stage (up to the 
time of gastrulation) as in the experiments of Spemann ('00'04) where such dissociation was effected by mechanical constriction along the potential embryo's longitudinal axis. It is 
only necessary to imagine that a chemical lesion of a moderate 
degree is sustained more or less along the germ's chief axis. 
Along this chemically altered area a rupture may take place 
owing to least resistance to osmotic pressure. Depending upon 
the extent of this passive 'fission/ a diprosopos, dicephalos 
or any diplopagus may result from the further development of 
the, thus partially, doubled primordium. The assumption of 
this chemical action, made already by Bataillon (I.e.), 14 does 
not necessarily apply to all duplicities of 'Nature' and not even 
to all double embryos in teleosts, for as I have already mentioned, the duplication of the embryonic anlage at any stage 
before the conclusion of gastrulation might be accomplished by 
the physical action of osmotic pressure only. And even the 
malformations of the components of the resulting duplicities 
might well be due to this factor alone. 15 


10 Rarely. 

11 This assumption is justified in view of the effects of osmotic pressure directly observed by Loeb (I. c.) in the sea-urchin. It should also be noted thatLoeb's 
method — decrease of external osmotic pressure by diluting the sea-water — excludes altogether the possibility of chemical action. 

12 In previous papers I ('15 and '16 b) have defined inhibition as a decrease 
of the germ's originally inherent 'chemomorphic' potentiality. This conception is based on the exceedingly suggestive hypothesis advanced in the last two 
or three decades by a number of biochemical authors and most recently presented in a very succinct and attractive form by Reichert ('14), according to which 
the development of an organism is a complex series of reactions in a stereochemical 
system. Reichert's paper came to my attention through the kindness of Prof. 
H. V. Wilson after the completion of the manuscript for the present paper. 

13 The theory of blastolysis was presented rather exhaustively in a former 
paper (Werber '16b). Owing, however, to misrepresentations which it suffered 
at the hands of certain authors the reader will; I hope, find that a repetition of 
its salient points in this paper is justified. 




In my experiments, however, substances have been added 
to the sea water which, owing to their chemical properties are 
toxic (i.e., injurious to the most important life processes), 
and which also, owing to their molecular weights have changed 
the tonicity of the sea-water. It is therefore not surprising that on examination of sections of some double monsters resulting from these experiments conditions are found which suggest 
that the duplication of the embryonic anlage was due to dissociation by the combined action of both chemical alteration 
and osmotic pressure (blastolysis) . 



14 Bataillon (I.e.) also assumes that in his experiments chemical alteration 
may have played a considerable part in the genesis of defects. 

15 As an example I am inclined to regard the frequently observed Salmonid 
double monsters. In a considerable number of trout double embryos in my 
possession (collected in a fish hatchery in the vicinity of Freiburg i. B.) examination of sections fails to disclose anything that would suggest chemical alteration as underlying the duplication of the germ and the accompanying deformities of the resulting double embryos. The nature of the alteration of the water 
in which these embryos have developed is, of course, unknown and the factors 
responsible for their developmental anomalies are, undoubtedly, left altogether to speculation. It seems not improbable, however, that in such cases 
the osmotic pressure of the water may be increased by the presence in it of 
some (otherwise perhaps not injurious) metallic salts, or possibly blastotomy 
here results from increased temperature or "old age" of the eggs (cf. pp. 
319-321 and 327). 




The following cases may serve as illustrative examples of the 
apparent effects of such action. 

In figure 11 is presented a 'parasitic' duplicity which in toto 
made the impression of a greatly deformed craniothoracopagus. 
Examination of sections practically confirms this diagnosis, 
except for the incompleteness in the duplication of the head. 

The transverse sections (6 m in thickness) being somewhat 
oblique, those farthest anteriorly contain more of the right than 
of the left side of the duplicity. A large lens is noted in them, 
an olfactory pit and a distorted, unilobed fore-brain. The 
lens (but no eye on this side) was observed already in toto and 
was mistaken for a 'free lens.' In further sections, however, 
there comes into view in apposition to it a very small optic 
cup (fig. 14). A very small, lens-like body (I) can also be observed in several sections at this level between the ill-differentiated optic cup and the large lens. On following the sections posteriorly the eye of the left side gradually comes into 
view (figs. 14 and 15), while the optic cup of the right side becomes more and more elongate and is noted still to be in direct 
connection with the brain, no optic stalk having been formed. 
In sections still further posteriorly this eye is very plainly seen 
to be broken up (dissociated) into several parts — one of which 
at the base of the brain with which it is connected, has made a 
feeble attempt towards differentiation of the retinal layers, rods and cones being discernible (fig. 15). At this level the 
brain has increased in size and presents a very distorted mass 
of nervous tissue in which cellular areas predominate. Still 
further posteriorly the buried optic cup of the right side is observed to gradually 'reconstitute' itself. It is very large at 
this level and its structure still betrays evidence of dissociation 
(ox. fig. 16). The brain increases in size with every successive 
section (fig. 17). Some parts of it give the impression of 
retinal tissue, but no certainty can be felt regarding the 
latter point. 



Fig. 1 (a and b) Anterior and posterior views of slightly conjoined twins, 
A the larger, B the smaller one, From acetone solution (40 cc. gram-molecular to 
50 cc. of sea-water), 19 days old, h., heart, pc, pericardial vesicle. 

Fig. 2 (a and b) Two views of slightly conjoined double embryo, A the larger, B the smaller one. From butyric acid solution (10 cc. of a 1/16 gram-molecular solution added to 50 cc. of sea-water), 13 days old, pc, pericardial vesicle. 

Fig. 3 From acetone solution (35 cc. gram-molecular tooOcc. of sea-water), 
14 days old. 

Fig. 4 From acetone solution (35 cc. gram-molecular to 50 cc. of sea-water), 
14 days old. 






The duplication of the brain becomes distinct at a level where 
the (dissociated) part of the larger component is on one side 
intimately fused with the medulla of the smaller component 
('parasite' — fig. 18). Two notochords and two (incomplete) 
alimentary tracts are also noted on the monster's right and left 
side respectively (figs. 17 and 18). Sections passing through 
the larger component's tail show distinctly (fig. 19) that the 
partial doubling of the central nervous system has come about 
through a splitting of one anlage by dissociation. 

A clue to the genesis of the described conditions is found in 
anterior sections. Here the epithelium is dissociated (d.e., 
fig. 14) in the region of the mouth and in a part of the blastoderm. The single oral cavity of the double monster is not 
continuous, but anteriorly partly occluded by the dissociated 
epithelium of the mouth (d.m., fig. 15) and posteriorly by the 
dissociated posterior part of the optic cup, while posteriorly 
to the latter the lumen of the mouth appears as a narrow slit, 
being 'plugged' by dissociated epithelium. The latter as well 
as all of the epithelium of the mouth consists of large, vesicular 
cells. 

Turning, now, to a consideration of the mode of formation 
of this monster, it would appear improbable that it has arisen 
from separated blastomeres and subsequent fusion during the 
formation of the embryonic bodies. The condition of the nervous system described above would seem to indicate clearly 
that at first one embryonic anlage has existed until probably 
about the time of formation of the embryonic body when the 
greater part has apparently been doubled by dissociation. 


As an other case where the duplication has apparently occurred at a late stage (during the formation of the embryonic 
body) may be regarded the embryo (mesocatadidymus?) presented 
in figure 13. 

This is a monster to which I attach much significance, because it most strikingly suggests the syngenesis of 'defects' 
with 'excess.' For its anterior part is highly deformed and 
defective while the greater part of the trunk and the tail — the 
former only partly — are duplicated, the spinal cord and notochord being multiple. 

Examination of sections shows these conditions in a very 
convincing manner. 

With the description of the defects of the head we shall not 
concern ourselves here, as it has a special bearing on the origin 
and differentiation of the lens and is therefore reserved for 
another paper. Suffice it to state, however, that the central 
nervous system is very defective throughout for about the anterior third of the body, i.e., to about the level of a plane passing transversely through the posterior part of the (only) 
pectoral fin. Just about from this level onwards posteriorly 
the spinal cord, in successive transverse sections, is observed at 
first to flatten and then to spread out more and more from side 
to side. It gradually increases in size and in many consecutive 
sections appears to be vacuolated and to exhibit evidence of 
cytolytic degeneration (fig. 20). The latter now becomes 
more and more conspicuous with each consecutive section (fig. 
21), cellular elements disappearing more and more, until in 
some sections not much more than the framework of supporting 
tissue is left. The nuclear debris of the chromatolyzed cells 
can be observed at quite a distance from this degenerated part 
of the cord. 

This has then, evidently, been the part which suffered the 
greatest injury from chemical alteration and subsequently 
presented a point of least resistance to the action of osmotic 
pressure. For from this level on the spinal cord divides at first 
into two and then into more parts. The notochord is also 
multiplied, while in sections at a still more posterior level the  body of the embryo is observed also externally to be partly 
duplicated (figs. 21 and 22). The musculature is not doubled; 
yet there is a decided increase of it in bulk. The alimentary 
tract is also single and even incomplete, some of its parts lacking altogether. In sections through about the posterior fourth 
of the embryo there appear just about the level where the intestine ends, two large cavities lined with endothelium whose genesis and morphological significance I am unable to interpret. 

The genesis of the chief morphological deviations of this 
embryo might be imagined in the following manner: 

The part of the embryo which was to form the head has sustained the greatest destructive alterations from the sojourn in 
the toxic solution. Hence the extreme malformation and the 
defects of this part of the body. The bilateral parts of the germring have apparently — owing to osmotic pressure — failed at 
first to fuse completely, this fusion occurring only later when 
some anlagen have, in this way, been doubled. The increase 
in osmotic pressure has also most likely contributed to the fragmentation and dispersion of some anlagen, as is evidenced by 
the condition of the notochord and the spinal cord. 16 

While the embryo is not a double monster in the strict sense 
of the word, it well illustrates the manner in which — according 
to Fischel' s suggestion — certain double embryos may arise 
during the formation of the embryonic body. It would also 
seem to offer strong support to our contention that the same morphogenetic factor — blastolysis — is apparently responsible for 
both the 'monstra per defectum' and the double monsters and 
other 'monstra per excessum.' 


Fig. 5 From acetone solution (35 cc. gram-molecular to 50 cc. of sea-water), 
14 days old. 

Fig. 6 (a and b) From acetone solution (35 cc. gram-molecular to 50 cc. of 
sea-water), 16 days old; i.e., 'isolated eye.' 

Fig. 7 From acetone solution (35 cc. gram-molecular to 50 cc. of sea-water), 
14 days old. 

Fig. 8 From acetone solution (35 cc. gram-molecular to 50cc.of sea-water), 
14 days old, pc, pericardial vesicle. 

Fig. 9 From acetone solution (35 cc. gram-molecular to 50 cc. of sea-water), 
one of the 'twins' amorphous, 14 days old, pc, pericardial vesicle. 

16 Neither verbal description nor the accompanying figures (of necessity 
limited in number) are adequate to give a complete picture of these conditions. 
To obtain an adequate idea of them and the dissociation that underlies them and 
is apparent in the sections, it is necessary to examine the latter in the series. 






The next example which we have chosen for the demon stration of the apparent syngenesis of both these deviations from 
the norm may now follow. 

In figure 12 is presented an egg with a large 'pericardial' 
vesicle, dense vascularization of the yolk-sac and a very curiously misshapen embryonic mass which in toto was with some 
doubt interpreted as a dwarfed and highly malformed double 
monster. Microscopic examination of sections fully confirms 
this interpretation. 

The first sections pass through the anterior part of the larger 
component only. They show (fig. 23) a dissociated tissue of 
an indifferent character in which are embedded two lenses, I and 
li. Further sections show that this tissue mass increases in 
density posteriorly and that it is the anterior, greatly dissociated, part of the single optic cup which 'eventually comes fully 
into view surrounding on all sides the larger one of the two 
lenses (fig. 24). The brain of this component is unilobed, very 
irregular in shape and unusually small, while the eye is very 
large. The unusual shape of the latter (fig. 25) as well as also 
the fact that the ill-differentiated layer of rods and cones does 
not (as it should) appear throughout the entire outer margin 
of the optic cup wall, but is observed for a considerable stretch to 
merge into the other layers of the retina, suggests that the eye 
has been formed out of a dissociated anlage. The above mentioned dissociated optic tissue mass anterior to the eye adds 
considerable weight to this evidence of blastolytic action. 

The smaller component appears first in the twelfth transverse 
section. It is also possessed of one eye only which is rather 
small anteriorly and very irregular in shape. In more posterior 
sections, however, this optic cup gradually increases to an unusually large size, (o.c, fig. 26) it being almost as large as the 
brain of the component. The latter is unilobed, very irregular 
in shape and relatively very large. 

On following the sections further the brain of the larger component is observed to increase to a very large size (figs. 26 and 
27). Cellular areas predominate in it and are strangely intermingled with fibrous areas. It is difficult to escape the impression that distortions of this nature can have resulted from 
anything but a disorganization of parts of the primordium and 
subsequent processes of regulation. 

At this level there can be observed in the larger component 
also a very small, vestigial ear vesicle (figs. 26 and 27). More 
posteriorly the notochord of this component is doubled, thus 
indicating dissociation of its anlage at an earlier stage of development. The (rudimentary) tail of this component comes 
into view in the last sections. A rudimentary tail is also observed in sections passing through the last part of the smaller 
component (fig. 27). 

The internal organs of both components are almost entirely 
obliterated. 

A point of interest is presented also by the epidermis of both 
components on the side on which they are conjoined. A cluster 
of large cells (d.e. figs. 25, 26 and 27) is noted here to come off 
directly from the epidermis. The large size of the cells would 
seem to indicate that having through dissociation lost their correlation with the epithelium of which they originally formed a 
part, there was no restriction to their expansive growth. In 
some sections the nuclei of these cells exhibit evidence of chromatolysis, thus suggesting that chemical alteration may have 
partly been responsible for dissociation (chemical blastolysis) . 

Turning, now, to the morphogenesis of this monster, we are 
justified, I believe, in assuming that it is a product of the combined action of, both osmotic and chemical, blastolysis. The 
former ('blastotomy') may by separating blastomeres (of the 
two-celled stage) have severed their mutual correlation, thus 
allowing each to develop independently. In the course of 
further development, however, the two, apparently not widely 
separated, embryonic primordia came into contact along their 
lateral surfaces where they coalesced. 





The deformities and great defects (cyclopia) exhibited by 
both components are due to loss of embryonic substance by the 
double primordia owing largely to destruction by chemical 
alteration. The parts surviving this destruction have after 
regulation developed into two dwarfed, highly defective bodies. 




Fig. 10 'Parasitic' double monster from acetone solution (35 cc. of grammolecular to 50 cc. of sea water), 14 days old. (Note the protruding, dissociated 
eyes of the 'autosite'.) 

Fig. 11 Parasitic double monster from acetone solution (35 cc. gram-molecular to 50 cc. of sea-water), 14 days old. 

Fig. 12 Dwarfed double monster from acetone solution (25 cc. gram-molecular to 50 cc. of sea-water), 15 days old, pc, pericardial vesicle. 

Fig. 13 Mesocatadidymus (?) from acetone solution (30 cc. gram-molecular 
to 50 cc. of sea-water), 26 days old. 



A number of other duplicities have been recorded which also 
are monstra et per defectum et per excessum (figs. 1 to 10). 
Their genesis is, in all probability, due to the same factors that 
have been assumed for the monsters above described. Thus 
in the monster illustrated in figure 9, presenting double embryos 
of strikingly unequal size and greatly deformed, one of the 
' twins' being amorphous, on microscopic examination indications 
are found of chemical alteration, while in the duplicity of figure 
6 in which an isolated head fragment with a well differentiated 
eye (ascertained by microscopic examination) is observed at 
a considerable distance from either of the 'twins', action of 
osmotic pressure is apparent already on examination in toto, 
if conclusions from analogous observations in invertebrates 
(cf. for instance, Loeb, I.e.) be permitted. 

The smaller one of the double embryos of figure 8 when examined in sections is observed to lack the anterior part of the 
head, the otic capsules appearing in the anterior-most sections. 
This is one of the most conspicuous defects of this component 
and, in my opinion, no error will be committed by assuming 
that the factor which brought about the duplication of the originally single embryonic primordium (osmotic pressure) may have 
contributed to the genesis of this and other defects by splitting 
off the chemically incapacitated parts. In the other double 
embryos recorded in our experiments which obviously are, at 
the same time also monstra per defectum microscopic examination of sections would, undoubtedly, furnish indications of the 
combined action of chemical alteration and osmotic pressure as 
underlying the doubling of primordia and the origin of the 
defects. 

==Theoretical Remarks and Conclusions==

As pointed out repeatedly in the preceding pages no direct 
evidence is available for this combined action of osmotic pressure and chemical alteration. It may, therefore, seem desirable to inquire in how far their assumption is justified. 

Nearly all duplicities of our experiments have resulted from 
the addition to sea-water (50 cc.) of a varied quantity (30 cc.  or 35 cc. or 40 cc.) of a gram-molecular solution (in distilled 
water) of acetone, only a very few resulting from the employment of butyric acid. Considering the molecular weights of 
sea-water on the one hand and of acetone on the other hand, it 
is at once evident that by the addition of the acetone solution 
to sea-water the latter was quite appreciably diluted. In 
other words, as far at least as this physical condition is concerned, 
our experiments with acetone are in principle similar to the 
experiments performed by Loeb (I.e.) on sea-urchin eggs by 
diluting the sea-water with fresh water. In both instances the 
osmotic pressure of the medium surrounding the eggs was lowered, although less so in my experiments than in those of Loeb. 
Since, in this way, the difference between the internal osmotic 
pressure (of the eggs) and the external osmotic pressure (of 
the surrounding medium) was modified in experiments which 
besides other monsters yielded a certain number of duplicities, 
there is no apparent reason to doubt that the origin of the 
latter is directly traceable to this modification. 

It might perhaps be wondered why relatively few double 
embryos have resulted from the above described treatment of 
the eggs. This, however, can, at least partly, be accounted 
for. The egg of Fundulus heteroclitus, well known for its hardiness, 17 is relatively little susceptible to physical insults, and it 
is possible that the dilution of sea-water by the addition to it 
of such quantities of acetone as I have employed, is not sufficient 
to cause osmotic blastolysis ('blastotomy' — Bataillon, I.e.) 
except in a very few eggs. Greater dilution of the sea-water 
by the addition of quantities of acetone greater than those employed in these experiments is, however, impracticable, for, in 
that case the degree of chemical action would be higher, and 
owing to it the mortality would, according to my experience, 
increase greatly and the few surviving eggs, having sustained 
so much chemical alteration, would result only in dwarfed, 
shapeless embryonic masses and fragments. The paucity of double monsters in our experiments can, accordingly, not serve 
as an argument against the justification of our assumption of 
osmotic pressure as an important factor underlying their 
morphogenesis. 

17 In a number of experiments performed in 1915 in which eggs of Fundulus 
heteroclitus were subjected to a strong centrifugal force, no alteration of development or any injury resulted. 



The second component factor of blastolysis, chemical alteration — is also largely inferred. However, this inference would 
seem to be well justified, if the action of acetone, partly solvent 
and partly as a precipitant of lecithine, is considered. Owing 
to the effect of this action some groups of cells may be entirely 
destroyed, while others may be only, more or less, chemically 
modified* and thus lose much of their inherent capacity for 
differentiation (inhibition) as well as of the power to resist the 
action of osmotic pressure. 

Besides the two factors mentioned above attention may be 
called to another factor which may play a contributory part to 
the effect we term blastolysis insofar as the latter may be facilitated by it. This is the age of the egg, counting from the time 
of its maturation. 

It has been found by several observers that the viability of 
the eggs and their ability to develop in a typical manner decreases gradually after their maturation. 

Thus Hertwig ('92) noted that unfertilized frog eggs retained 
(for some time after the animal had been killed) in the uterus 
(on ice), showed with each consecutive day a greater tendency 
to abnormal development, which in many of them resulted in 
'spina bifida.' Like results have been obtained by him also 
if female frogs were separated from males long enough for the 
eggs to become over-mature. 

Conklin's ('97) observations on the eggs of Crepidula are very 
striking and may therefore be best presented in his own words 
(p. 30): 

. . . . when the adult Crepidulas are brought to the laboratory, 
and kept in the best possible conditions, the percentage of these abnormalities increases, and when the egg capsules are removed from 
the mantle cavity of the mother and. kept in dishes of sea-water, the 
monstrosities increase to such an extent that after a few days not a 
single normally developing egg or embryo can be found. 


Bataillon (I.e.) has observed a very marked tendency to 
fragmentation and formation of double embryos in eggs of 
Petromyzon which had been retained in the females for several 
days after stripping. Since no modification of the environment 
whatsoever was employed in this case, Bataillon speaks of 'blastotomie spontaneV as the factor responsible for duplication. 

Of the causes underlying this apparent deterioration of 'stale 
eggs' Goldfarb ('16, '17 a and b) has made a systematic study. 
He finds that it is apparently due to a progressive increase in 
their permeability and to the thus resulting rise in the rate of 
their oxidation, as postulated long ago by J. Loeb. 

I am inclined to attribute considerable importance to this 
factor in our experiments in which, usually, eggs of several 
females were fertilized together before they were subjected to 
the action of the environmental modification. In other words, 
the material of our experiments was of a varying viability and 
varying susceptibility, the 'younger' eggs being less susceptible 
than the 'older' ones. This variability cannot well be obviated 
in any experiments of an explorative nature. For a single 
female of Fundulus heteroclitus will not yield eggs enough for 
a coherent series of experiments in which the same environmental modification is employed, but differing in each experiment in degree or in the stage at which the eggs were subjected 
to it. Besides, it is well known that not all eggs spawned by, 
or stripped from, a female at the same time are of the same 
'maturation age.' It is a common experience to find mature 
and immature eggs in the same female during the spawning 
season, a fact which well indicates that the mature eggs have not 
all matured at the same time. 

To this variation in degree of susceptibility of the eggs may 
be due the enormous range in variation in the end-products of 
development — from apparently perfectly normal embryos to 
monstrosities of a most bewildering kind. The duplicities which 
were recorded in our experiments are an incident in this almost 
endless 'series.' They result primarily from duplication of the 
embryonic primordium by blastolysis. And they as well as 
all other monstrosities are expressions of the variation in the degree of this action, for they, too, range from apparently symmetrical, well formed double embryos through various degrees 
of deformation to highly defective and grossly malformed double 
monsters (figs. 1 to 13). 

Among some other points which would also seem to call for 
explanation is the often recorded unequal size of the components 
of a duplicity where two whole slightly conjoined or entirely 
separate embryos have developed (figs. 1, 2, 3, 4, 6, 7, 8, 9, 10, 
ll). 18 In my opinion the only explanation possible for this 
phenomenon is that apparently even less than one-half of the 
entire embryonic primordium is still totipotent in teleosts just 
as Eataillon (I.e.) has found this to be the case in Petromyzon. 
In the case of our double embryos I would further suggest that 
after the splitting of the germ into two parts one of them has 
apparently sustained further blastolytic lesion which, however, 
while dwarfing it in size, has not diminished its totipotency. 

In this connection it may be well to consider also the sometimes very striking differences in degree of malformation between the components of a duplicity (figs. 2, 4, 8, 9, 10 and 
11). In accordance with our interpretation of the genesis of 
malformations (Werber, '1G) it will have to be assumed that the 
more deformed component was subject to blastolytic action in 
a higher degree than the less deformed one. The above mentioned difference in size between components of the same duplicity is just another effect of this simple principle. I am, however, unable to account for these differences in degree of action 
on duplicates of the same germ. 

Yet these differences (both in size and degree of malformation) between components of the same duplicity I regard 
as important insofar as they would seem to furnish a clue to 
the morphogenesis of the so-called 'parasitic double monsters'. 
For, granting an - early and intimate secondary fusion of primordia duplicated by blastolysis, and that one of these double germs has sustained much greater injury than the other, the 
end-product of further cfevelopment will be a more or less well 
formed (sometimes even much deformed) embryo (the 'autosite') with a sort of 'parasitic appendage'-- the 'parasite.' 19 
This conception of the manner of formation of the 'parasitic' 
duplicities differs essentially from Wilder's ('08, pp. 363-364, 
ff.), for no resort is made to the improbable secondary degeneration of one (whole) of the already formed components of the 
diplopagus owing to — what briefly may be called — physiological dominance of the other component. 20 


18 Similar observations on very young double embryos have been made also by 
Klaussner ('90) in the chick and in the lizard Lacerta viridis and by Dareste 
('91) in the chick, while one such double embryo of the chick is in my 
possession. 


It may now be questioned in how far our conclusions regarding the origin of the duplicities in our experiments can be extended to those occurring in nature in other vertebrates and 
particularly in mammals. "While I am, of course, disinclined 
to do so unconditionally, I think that with some restrictions 
this extension may be regarded as permissible. For the oftassumed possibility, if not probability, that in nature, too, the 
primary causal agent of atypical development is a chemical 
one (probably due to some disturbances of metabolism during 
which substances are produced by the body otherwise foreign 
to it) can not be denied. The human organism — and possibly 
that may apply to other mammals and to Sauropsidae as well — 
is subject to disturbances of metabolism of both a serious and 
a relatively harmless nature. To the former may be counted 
nephritis, diabetes, jaundice and other diseases of metabolism, 
while as a relatively harmless, temporary disturbance of metabolism may be viewed the presence in the organism of certain 
substances of fatigue after muscular exertion (for instance, lactic 
acid or monopotassium phosphate) and perhaps some other products of transitory abnormal conditions. While the substances 
of the former owing to their continued more or less severe action 21 on the ovum of a female suffering from any of these 
diseases may cause either its death and subsequent expulsion 
or any conceivable monstrosity, the latter, if altering the tonicity only of the blood, may by (osmotic) blastotomy produce 
slight defects or duplication of the embryonic primordium and 
thus in the latter case be responsible for the development of 
monozygotic twins, or any monozygotic, more or less well 
formed, 'symmetrical' duplicity. 


19 Fischel (I.e., p. 290) also suggests that these 'parasitische Anhange' develop from primarily rudimentary anlagen. 

20 The differences between the 'parasitic appendages' and the foetal inclusions ('teratomata', 'embryomata', etc.) I regard, with Fischel, (I.e., pp. 290299 ff) as secondary only. A discussion of this subject is deferred to a future 
publication. 



It might further be questioned whether the analysis of the 
morphogenesis of our experimental fish duplicities as attempted 
in the preceeding pages on the basis of FischePs theory is applicable to the higher vertebrates and mammals. 

Sobotta ('14) has already pointed out that while it is very 
doubtful whether the first two blastomeres are equipotent and 
totipotent in mammals it may be regarded as hazardous to 
extend to the latter the conclusions reached regarding the 
morphogenesis of duplicities in lower animals. However, occasioned primarily by the recent researches of Fernandez ('09) 
and Newman and Patterson ('10) and Patterson ('13) he suggests 
hypothetically a stage (the ' embryonic blastomere' of the fourcelled stage) whose parts after the next cleavage might be 
totipotent and thus capable of giving rise to monochorionic 
double embryos if separated. 22 Undoubtedly, in view of the 
meager data available for the earliest development in mammals, 
generalizations must be made with extreme caution. Yet, 
whatever the mammalian embryo-forming equivalent of the 
ovum of amphibia (or other animals in which the first blastomeres are totipotent) may be, there is, as Fischel (I.e.) has already pointed out, no apparent reason to expect that its prospective potency should be lower than that of the latter. 

21 Depending partly upon the 'maturation age' of the ovum at the time of its 
fertilization. 

22 For Tatusia novemcincta, on which Patterson's ('13) investigations were 
carried out, Sobotta assumes the separation of the four cells resulting from the 
second cleavage of the hypothetical 'embryonic blastomere.' Pending further 
investigations on the earliest stages of development in armadillo I should regard as questionable the analogy between the polyembryony in the latter and the 
occasional instances of 'twinning' in other mammals. 


Granting, however, the possibility that up to the time of 
gastrulation the mammalian (and Sauropsid) ovum is 'capable 
of being divided into two or possibly more equi- and totipotent 
parts, it would remain to be shown that the parts thus separated 
may, as in Petromyzon and teleosts, be able to form any kind 
of duplicity, depending upon the degree of separation, the distance between the parts separated and the angle of their respective convergence. , There is reason to believe that no differences should exist in this regard between the teleost ovum 
on the one hand and the Sauropsid and mammalian ovum on 
the other hand. For, as Fischel sets forth (I.e., p. 274), the 
processes leading to the formation of the embryonic body (separate anlagen for the head and trunk, the latter formed by concrescence of two lateral parts) being in principle very much the 
same in all vertebrates the morphogenesis of double monsters 
in teleosts may — at least with certain restrictions of secondary 
significance — be extended to amniotes and even to man. 

These conclusions are, obviously, tentative and — in default of 
experimental tests in mammals — they will probably remain so 
for an indefinitely long time. Their advantage, however, besides offering a plausible explanation, is that they may help 
to keep alive further inquiries into the, so far inapproachable, 
problem of diplogenesis ('twinning') in amniotes. 

It is for this reason mainly of methodological advantage that, 
— in default of evidence to the contrary— I am inclined to hold 
to the views above expressed in preference to Wilder's speculations- 3 on the subject. For, while I can find no fault with this 
writer's reasoning, I consider that his separation of the double 
monsters (sensu stricto) from well formed, symmetrical duplicities ('orderly beings — cosmobia') and the assumed mysterious 
origin of the latter by a sort of mutation due to ' germinal variation' are both unnecessary and arbitrary. The lack of deformities in 'Cosmobia' as well as in viable monozygotic twins 
would rather seem to indicate that while they may be developmental products of primordia duplicated by blastotomy, they have apparently sustained no destruction due to this or any other 
factor. The symmetry in anatomical details observed in 'Cosmobia' may, I venture to say, be only an incident — symmetrical 
blastotomy and the subsequent undisturbed development of 
the primordia which havej thus incidentally, remained equipotent. Conversely we may assume that where (osmotic) 
blastotomy or blastolysis (both osmotic and chemical) causes 
destruction of some areas of the germ, any monster and — if the 
germ has also been split into two more or less equivalent parts — 
any deformed duplicity may result from further development. 
The distinction between these two kinds of duplicities is, 
accordingly, mainly one of degree; it is a quantitative difference rather than a qualitative one and it may depend upon the 
specific action (either chemical or physical or both chemical 
and physical) of the (probably chemical) agent primarily responsible for the deviation from the typical course of development.

23 A view similar to that of Wilder's seems to have been advanced also by Bolk 
('06), whose paper is, unfortunately, inaccessible to me. 


 

Moreover, this quantitative difference may, as pointed out 
above, depend also upon the 'age' of the egg, the 'younger' 
eggs being hardier and more capable of surviving blastotomy 
without sustaining fatal lesions, while the 'older' ones with a 
tendency to cytolytic fragmentation may apparently be (in a 
varying degree) more subject to such lesions produced by blastotomy or blastolysis. 

The 'old age' of the egg is, undoubtedly, a germinal variation, 
but this 'variation' is in the nature of a pathological change and 
the causes underlying the latter may well be within the bounds 
of discovery, as is suggested by the recent researches of Goldfarb (I.e.). The germinal variation, however, assumed by 
Wilder is not a pathological condition and being nothing else 
that could intelligibly be defined, it automatically puts a stop 
to further inquiry into the primary causes underlying diplogenesis in nature. 

While I am not at all inclined to underestimate the almost 
insuperable difficulties presented by the problem, I cannot on 
the other hand subscribe to Wilder's skepticism voiced in the 
following characteristic remark ('08, p. 428) : 


. . . . even though the monsters resulting from the present 
or future experiments, in which the cause is applied after the formation of the germ and is therefore secondary, be found upon careful 
examination to be definite Cosmobia, it will prove only that such 
monsters can be thus produced and not that Nature does produce 
them in the same way. 

It is, obviously, difficult to deny the justifiability of this 
argument, but on the other hand — I think Professor Wilder 
will admit — it practically amounts to questioning the value of 
many explorative experiments in biology. 

May 30, 1917. 

==Literature Cited==

(For a more complete list of the extensive literature on the subject the reader 
is referred to the chapter on double monsters in Schwalbe's textbook and to 
Hiibner's article.) 

Bataillon, E. 1901 La pression osmotique et les grands problemes de la 

Biologie. Arch. f. Entw.-Mech., vol. 11. 
Bolk, L. 1906 Dubbelmonstra, hun Classificatie en Ontstaan. Genees
kundige Bladen uit Klinik en Laboratorium. 12. Reeks., Nr. 9 en 

10. Haarlem. (Quoted from Hubner.) 

Conklin, E. G. 1897 The embryology of Crepidula. Jour. Morph., vol. 13. 

Driesch, H. 1893 Entwicklungsmechanische Studien. III-VI. Zeitschr. f. Wiss. Zool., vol. 55. 

Fernandez, M. 1909 Beitriige zur Embryologie der Giirteltiere. Zur Keim
blattinversion und spezifischen Polyembryonie der Melita (Tatusia hybrida Dem.). Morpholog. Jahrb., vol. 39. 

Fischel, A. 1898 Experimentelle Untersuchungen am Ctenophorenei, I-IV, Arch.' f. Entw.-Mech., vols. 6 and 7. 

1902 tjber den gegenwartigen Stand der experimentellen Teratologic Verhandlg. d. deutschen patholog. 

Gesellsch., Berlin, 1903. S. 255-357. Referat gehalten auf der 5. Tagung der deutschen patholog. 

Gesellsch., Karlsbad, 22-26 Sept., ^902. 


Gerlach, L. 1882 Die Entstehungsweise der Doppelmissbildungen bei den hoheren Wirbeltieren. Stuttgart. 

Goldfarb, A. J. 1916 Experimental studies upon stale germinal products. Carnegie Institution of Washington. Year Book No. 14, for the year 1915. 

1917 a Experimental studies upon the aging and death of germ-cells. Carnegie Institution of Washington. Year Book No. 15, for the year 1916. 

1917 b The effects of ageing upon germ cells and their development. Proc. of the Society for Exp. Biol, and Medicine. Eighty-first meeting, February 10. 

Herlitzka, A. 1895 Contributo alio studio dello capacita evolutiva dei due 
primi blastomeri noil' uovo di tritone. Arch. f. Entw.-Mech., vol. 2. 
1897 Sullo svilupo di embryoni completi da blastomeri isolati di 
uova di tritone (Molge cristata). Arch. f. Entw.-Mech., vol. 4. 

Hertwig, O. 1892 Urmund und Spina bifida. Arch. f. mikr. Anat., vol. 39. 

1903 Missbildungen und Mehrfachbildungen, the dutch Stoning der ersten Entwicklungsprocesse hervorgerufen werden. Handbuch der vergleichenden und oxperimentellen Entwiclungslehre. Jena. pp. 967-989. 

Hubner, H. 1911 Die Doppelbildungen des Menschen und der Tiere. Lubarsch und Ostertag's Ergebnisse der allgemeinen Pathologic und pathologischen Anatomie der Tiere. 

Klaussner, F. 1890 Mehrfachbildungen bei Wirbeltieren. Eine teratolo
gische Studie. Mtinchen. 

Loeb, J. 1895 Beitrage zur Entwicklungsmechanik der aus einem Ei entstandenen Doppelbildungen. Arch. f. Entw.-Mech., vol. 1. 

Mall, F. P. 1908 A study of the causes underlying the origin of human monsters. Jour. Morph., vol. 19. 

Morgan, T. H. 1893 Experimental studies on teleost eggs. Anat. Anzeiger, vol. 8. 

Newman, H. H. and Patterson, J. T. 1910 The development of the ninebanded armadillo. Jour. Morph., vol. 21. 

Newman, H. H. 1917 The biology of twins. The University of Chicago Press. 

Patterson, J. T. 1913 Polyembryonic development in Tatusia novemcincta. Jour. Morph., vol. 24. 

Rauber, A. 1879-1880 Formbildung und Formstbrung in der Entwicklung 

von Wirbeltieren. Morpholog. Jahrb., vol. 5 and 6. 

Reichert, E. T. 1914 The germplasm as a stereochemic system. Science, N. S., vol. 40, No. 1036, November 6. 

Schtjltze, O. 1894 Die kiinstliche Erzeugung von Doppelbildungen bei Froschlarven mit Hilfc abnormer Gravitationswirkung. Arch. f. Entw.-Mech., Bd. 1. 

Schwalbe, E. 1907 Die Doppelbildungen. In: Die Morphologie der Missbildungen des Menschen und der Tiere. II. Teil. 

Sobotta, I. 1914 Eineiige Zwillinge und Doppelmissbildungen des Menschen im Lichte neuerer Forschungsergebnisse der Siiugetierembryologie. Studien zur Pathologic der Entwicklung, herausgegeben von R. Mayer und E. Schwalbe, vol. 1. 

Spemann, H. 1900 Experimentelle Erzeugung zweikopfiger Embryonen. Sitzber. d. phys.-med. Ges., Wiirzburg. 

1901 Experimented crzeugte Doppelbildungen. Verb.. V. Intern. Zool. Kongress. Berlin. 

1901, 1902, 1903 Entwicklungsphysiologische Studien am Tritonei. I, II u. III. Arch. f. Entw.-Mech., vol. 12, 15 and 16. 

1904 tJber experimented crzeugte Doppelmissbildungen mit zyklopischem Defekt. Zool. Jahrb., Suppl., Bd. VII. Festschr. f. 
Weissmann. 


Werber, E. I. 1915 Experimental studies aiming at the control of defective 

and monstrous development. A survey of recorded monstrosities 

with special attention to the ophthalmic defects. Anat. Rec, vol. 9. 

1916 a Blastolysis as a morphogenetic factor in the development of monsters. Proc. Amer. Ass. Anat. in Anat. Rec, vol. 10. 

1916 b Experimental studies aiming at the origin of monsters. I. 

An etiology and an analysis of the morphogenesis of monsters. Jour. Exp. Zool., vol. 21. 

Wetzel, G. 1895 Uber die Bedeutung der zirkularen Furche in der Entwick
lung der Schultze'schen Doppelmissbildungen von Rana fusca. Arch. 

f. mikrosk. Anat., Bd. 46. 

1896 Beitrag zum Studium der kiinstlichen Doppelmissbildungen  von Rana fusca. Inaug.-Diss. 

Wilder, H. H. 1904 Duplicate twins and double monsters. Am. Jour. Anat.,  vol. 3. 

1908 The morphology of Cosmobia. Speculations concerning the significance of certain types of monsters. Am. Jour. Anat., vol. 8. 

Wilson, E. B. 1893 Amphioxus and the mosaic theory of development. Jour.  Morph., vol. 8. 


==Plates==


PLATE 1 

EXPLANATION OF FIGURES 

14, 15 and 16 Transverse sections through 'parasitic double monster' of 
figure 11. L., lens.; I., small lentoid body; b., brain; e., anterior-most part of 
eye; d.e., dissociated epidermis; o.c, optic cup; d.o., dissociated optic cup; 
d.m., dissociated epithelium of mouth; y., yolk. X 105. 


PLATE 2 

EXPLANATION OF FIGUKES 

17, 18 and 19 More posterior sections through the embryo of figure 11. b., 
brain; n., notochord; i., intestine; d.m., dissociated epithelium of mouth; m.o., 
medulla oblongata of 'parasite'; s.c. spinal cord of 'autosite'; s.c. t , spinal cord of 
'parasite'; y., yolk. X 105. 


PLATE 3 

EXPLANATION OF FIGURES 



20, 21 and 22 Transverse sections through embryo of figure 13, n., notochord; 
s.c, spinal cord; d.s.c, dissociated and partly 'degenerated' spinal cord; s.c.i, 
s.c.t and s.e. 3 , dissociated, multiple spinal cord, cavities lined with endothelium. 
X 105. 




PLATE 4 

EXPLANATION" OF FIGURES 

23, 24, 25, 26 and 27 Transverse sections through dwarfed double embryo of 
figure 12. 6 1( brain of the larger component; b 2 . brain of the smaller component; 
h, and h lenses in anterior-most, greatly dissociated part of the eye of the larger 
component; I., lens (smaller component); e., eye; d.e., dissociated epidermis; 
7n.o., medulla oblongata of smaller component; e.v., rudimentary ear vesicle; 
o.c, posterior-most, greatly dissociated part of the optic cup of the smaller 
component; p., plasma in pericardial vesicle; /., tail of smaller component. 
X 105.

==Plates==

PLATE 1

EXPLANATION OF FIGURES

14, 15 and 16 Transverse sections through 'parasitic double monster' of figure 11. L., lens.; I., small lentoid body; b., brain; e., anterior-most part of eye; d.e., dissociated epidermis; o.c, optic cup; d.o., dissociated optic cup; d.m., dissociated epithelium of mouth; y., yolk. X 105.



PLATE 2

EXPLANATION OF FIGUKES

17, 18 and 19 More posterior sections through the embryo of figure 11. b., brain; n., notochord; i., intestine; d.m., dissociated epithelium of mouth; m.o., medulla oblongata of 'parasite'; s.c. spinal cord of 'autosite'; s.c. t , spinal cord of 'parasite'; y., yolk. X 105.



PLATE 3

EXPLANATION OF FIGURES



20, 21 and 22 Transverse sections through embryo of figure 13, n., notochord; s.c, spinal cord; d.s.c, dissociated and partly 'degenerated' spinal cord; s.c.i, s.c.t and s.e. 3 , dissociated, multiple spinal cord, cavities lined with endothelium. X 105.





PLATE 4

EXPLANATION" OF FIGURES

23, 24, 25, 26 and 27 Transverse sections through dwarfed double embryo of figure 12. 6 1( brain of the larger component; b 2 . brain of the smaller component; h, and h lenses in anterior-most, greatly dissociated part of the eye of the larger component; I., lens (smaller component); e., eye; d.e., dissociated epidermis; 7n.o., medulla oblongata of smaller component; e.v., rudimentary ear vesicle; o.c, posterior-most, greatly dissociated part of the optic cup of the smaller component; p., plasma in pericardial vesicle; /., tail of smaller component. X 105.





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