Paper - The relation of the growing Mullerian duct to the Wolffian duct and its importance for the genesis of malformations

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Gruenwald P. The relation of the growing Mullerian duct to the Wolffian duct and its importance for the genesis of malformations. (1941) Anat. Rec. 81: 1-19.

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This historic 1941 paper by Gruenwald described Müllerian duct (paramesonephric duct) and the Wolffian duct (mesonephric duct) development.

Also by this author: Gruenwald P. On the developmental mechanics of the urogenital system in chicken (Zur Entwicklungsmechanik des Urogenital systems beim Huhn). (1937) Arch f Entw-mechan. 136: 786–813.

Gruenwald P. The relation of the growing Mullerian duct to the Wolffian duct and its importance for the genesis of malformations. (1941) Anat. Rec. 81: 1-19.

Gruenwald P. The normal changes in the position of the embryonic kidney. (1943) Anat. Rec.

Gruenwald P. Basis of regenerative and pathologic growth in the uterus. (1933) Archives of Pathology. : 53–65.

Modern Notes: paramesonephric duct | mesonephric duct

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General: 1901 Urinogenital Tract | 1902 The Uro-Genital System | 1904 Ovary and Testis | 1912 Urinogenital Organ Development | 1914 External Genitalia | 1921 Urogenital Development | 1921 External Genital | 1942 Sex Cords | 1953 Germ Cells | Historic Embryology Papers | Historic Disclaimer
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The Relation of the growing Mullerian Duct to the Wolffian Duct and its Importance for the Genesis of Malformations

Peter Gruenwald

Department of Anatomy, The Chicago Medical School, Illinois

Three Text Figures And Two Plates (Twelve Figures)

This work was aided by a grant from the Emergency Committee in Aid of Displaced Foreign Medical Scientists.


The development of the müllerian duct is usually described as occurring in two phases, the first of which is the establishment of the funnel-shaped abdominal ostium from the coelomic epithelium of the urogenital ridge, whereas the second phase is represented by the growth of the duct proper from the funnel downward to the cloaca, at the side of the wolffian duct. The development of the ostium was brought in connection with pronephric funnels by various authors; this part of the formation of the miillerian duct will not be discussed here.

The second phase was described by early investigators as being accomplished by a longitudinal splitting of the wolffian duct in all vertebrates; the miillerian duct. was said to arise from the lateral (or latero-ventral) half. Later, at about the end of the nineteenth century, the general opinion shifted and most of the authors, while still admitting splitting of the wolffian duct in selachians, described the development of the müllerian duct in higher vertebrates as an independent lengthening of a bud growing caudad from the bottom of the funnel. The wolffian duct was now considered by some authors as not concerned with the development of the müllerian duct at all, by others as a mere guide for the latter. Hertwig, in his textbook, gives a comprehensive review of both opinions, favoring the more recent one, but admitting that several reports in favor of the older conception were still undisputed. Felix, in his famous contribution to Keibel and Mall’s Manual (712), says about the miillerian duct: ‘‘Its growth results entirely from its own forces, the outgrowing tip being always free and connected neither with the coelomic epithelium nor the primary excretory duct; only poorly preserved or poorly fixed preparations could deceive one on this point’’ (p. 913). Subsequent textbooks reflect this opinion with the exception of that of Brachet, Daleq and Gérard (’35), where it is stated that the portion of the miillerian duct in question develops by closure of the longitudinal groove extending caudally from the ostium. According to this description, only the very last, short portion of the duct grows in length by itself in order to reach the urogenital sinus. Study of serially sectioned mammal and chick embryos, as well as previous experiments on chick embryos (Gruenwald, ’37) reveal no support whatsoever for this opinion; therefore, it will not be considered in the following discussion.

The description of the development of the müllerian duct as an independent outgrowth from the ostium has been given in almost all textbooks of the last decades, and has generally been considered correct. For a long period of time no descriptive work was done concerning this point. In 1937, Gruenwald found by experiments in chick embryos that the growth of the miillerian duct must be influenced by the wolffian duct, because if the latter was interrupted, the miillerian duct did not grow caudad beyond the point of interruption. This was interpreted as a proof of the supposition that the wolffian duct serves as a guide for the growing miillerian duct. Later it was pointed out that the comparatively common incidence of certain syndromes of malformations in man makes it highly probable that the same relation exists in the human embryo (Gruenwald, ’38). In fact, several investigators had previously been led to suspect this relation by the conditions found in developmental anomalies.

Recent study of very well preserved and stained serial sections suggest a more intimate relation between the wolffian and miillerian ducts than is generally believed to exist. These findings will shortly be described and discussed in this paper.

Material and Methods

Only excellently preserved and stained serial sections of human and chick embryos were used for this investigation, particularly in view of Felix’ above quoted remark cautioning against misleading results from poorly preserved material. As it seemed most important to show the basal membranes of the structures in question, only azan stain or silver impregnation of the reticulum fibers (method of Gomori, ’39) were used. The former gives more information about the cells themselves in addition to a good representation of the connective tissue, whereas the latter gives the sharper and more complete pictures of connective tissue fibrils, and particularly the basal membranes. All embryos were fixed in Bouin’s fluid, the sections were 8 or 10p in thickness.

In order to facilitate the comparison of stages, the lengths of the miillerian ducts of the human embryos were measured. A differentiation was made between a portion free of the wolffian duct (i.e., with mesenchymal cells separating both), another one attached to it, and a third portion in which wolffian and miillerian ducts are fused with no basal membrane between them. Wherever the direction of sections was approximately at right angles with the ducts, the length was determined by counting the sections; when this was not possible, measurements were taken from graphically reconstructed projections of the ducts on a sagittal plane. Deviations in mediolateral direction are negligible in all embryos with the exception of the two oldest ones. In these two instances the length was estimated. This estimation concerns only the portion of the miillerian ducts free of the wolffian ducts, the exact length of which is not important for our considerations. These measurements are combined in a graph in figure 1.

From some of the serial sections of chick embryos graphic reconstructions were obtained showing wolffian and miillerian ducts and the adjacent thickened coelomic epithelium (tubal ridge), projected on a frontal plane (figs. 2 and 3). All reconstructions were made by measuring with an eyepiece micrometer and combining the measurements on millimeter paper.



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9 mm. ” ”

9.5 mm. ” ” 10.5 mm. ” ” 1 12 mm. ” ” 1, 6 13.5 mm, 104 ” 1, 4, 5 14. mm, Su silver 1, 7 15.5 mm. 10 w azan 1, 8, 9 18 mm. ” ” 1, 10 20 mm ” ” ] 21 mm ” ” 1 21 mm. ” ” 1 25 mm. ” ” 1, 11 29 mm ” ” 1 380) mm. ” ” 1, 12

2 mm, ”? >? Chick: 4d Su silver 4d 5h ” azan 13 4d 12h ” ” 4d 2th ” ” 5a ” azan, silver 5d oh ” silver 2, 14 5d 12h ” ” 15 5d 18h ” ” 3

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10.5 le 15,5 14 135 le 2 21 21 2 2 30

Fig.1 Diagram showing the length of the three portions of the growing miillerian ducts in human embryos designated by length in millimeters. Portion separated from the wolffian duct by mesenchyme, black; portion separated by basal membrane only, cross hatched; portion fused with wolffian duct, clear.

Fig.2 Graphic reconstruction of the wolffian and miillerian ducts and the coelomic epithelium (tubal ridge) of a chick embryo of 5 days and 6 hours. Mesenchyme and basal membranes: black; miillerian ducts: stippled. The lateral border of the wolffian duct was arbitrarily taken as a straight line.

Fig. 3 Similar reconstruction based on an embryo of 5 days and 18 hours,


In the three earliest human embryos listed in table 1, the area of origin of the millerian duct differs from the rest of the mesonephric ridge by the taller cells of its epithelium which just begins to form a funnel-shaped invagination. The wolffian duct lies close to this portion of the epithelium, but is clearly separated from it throughout.

In the 10.5 mm. embryo, a very short miillerian duct exists on the left side, being closely applied in its entire length to the wolffian duct. Both are separated by distinct basal membranes which fuse in the area of contact. The right miillerian duct is slightly longer and shows a later stage also in its relation to the wolffian duct (fig. 1). All three of the above mentioned portions may be found here. The cranial] one, separated from the wolffian duct by mesenchyme, is very short (less than 20). In the caudal portion, characterized by direct contact with the wolffian duct, the miillerian duct loses its lumen and continues as an obtuse wedge of cells covering the lateral aspect of the wolffian duct within the latter’s basal membrane. The 12-mm. embryo, in contrast, shows the left miillerian duct farther developed than its fellow on the right side. The latter shows two portions, a cranial one that is separated from the wolffian duct by mesenchyme, and a caudal one which comes close enough to the wolffian duct to allow the basal membranes to fuse. No direct contact of the epithelia of both ducts was found on this side. On the left side, all three parts can be distinguished, very much as on the right side of the 10.5-mm. embryo described before. The two cranial portions correspond to those on the right side, in regard to the relation of the two duets as well as in their lengths. Here, however, a short third portion is present in which the cells of both ducts are enveloped in a common basal membrane, and at one point cells of the millerian duct almost reach the lumen of the wolffian duct (fig. 4). The lumen of the miillerian duct disappears at the boundary of the second and third portions and the end of the duct forms, as in the embryo described before, a solid wedge between the epithelium and the basal membrane of the wolffian duct.

Tn all embryos from 13.5 to 29 mm. both miillerian ducts show the three divisions as described on one side in the 10.5 and the 12-mm. embryos. Figure 1 indicates that the two caudal portions grow fast and reach their greatest lengths in the 18.5-mm., 14-mm., and 15.5-mm. embryos. In the older specimens they are slightly shorter again. The cranial portion, separated from the wolffian duct by mesenchyme, increases continuously in length at the same rate as the two cranial divisions move caudad. At each particular level we find the Mullerian duct first as a solid wedge between the epithelium and the basal membrane of the wolffian duct, then acquiring a lumen, and finally we see it being separated from the wolffian duct by basal membranes and an increasing amount of mesenchyme.

Most interesting in this developmental process is the appearance of the solid wedge at the caudal end of the miillerian duct. We find here a great variety of pictures in different embryos, as illustrated by figures 4 to 11. The diameter of the miillerian duct at this level is usually greater than, or at least as great as that of the wolffian duct. The miillerian epithelium, having no lumen and no separate basal membrane, appears on cross sections as a solid bulge of the wall of the wolffian duet where it is nearest to the free edge of the mesonephrice ridge. In some instances millerian epithelium surrounds the wolffian duct along half of its circumference (fig. 5). The lumen of the wolffian duct is, as a rule, entirely surrounded by wolffian cells which may, however, be considerably lower in the area of contact with the miiJlerian wedge than along the rest of the circumference (figs. 6, 8, 10). In some instances, as in our 12-mm. and 13.5-mm. embryos (figs. 4, 6), miillerian cells almost reach the lumen of the wolffian duct.

Nagel, who gave an essentially correct description of these relations as early as 1889, stated that wolffian and miillerian cells are always clearly distinguishable, and that no intermediate cel] types can be found. This seems to be true as long as low magnifications are used. However, when we examine each cell individually under high magnification, and try to distinguish it as either wolffian or miillerian, we soon find difficulties with some of them. There can be no doubt that a few cells have ill-defined characteristics, although it must be admitted that their number is small, in fact smaller than we would expect, assuming that one of the two epithelia develops from the other. The wolffian duct caudal to the end of the miillerian duct is histologically of the same structure as the cranial portion. Only in the very last part near the urogenital sinus the duct is, shortly before being reached by the wedge, more similar to the miillerian than to the wolffian type of differentiation (29-mm. embryo). When the miillerian duct has reached the urogenital sinus, its separation from the wolffian duct continues until the area of direct contact of their epithelia disappears. This is almost accomplished in the 30mm. embryo (figs. 1, 12), and fully completed in the 32-mm. embryo.

Since experimental evidence of developmental dependence of the miillerian duct on the wolffian duct was obtained in chick embryos, it was of particular interest to investigate the morphological relations of both ducts in that species, too. Nine embryos of the fifth, sixth and eighth day of incubation, treated with the same technic as the human specimens, were used for this purpose (table 1). The pictures seen in these embryos (figs. 18, 14, 15) are essentially like those just described in the human series. During the early part of the fifth day the miillerian ducts begin their caudal growth from the later abdominal ostium. They are from the very beginning closely attached to the wolffian ducts, but, just as in human embryos, at first separated from the latter by basal membranes. During the sixth day of incubation the miillerian ducts reach a considerable length, and now the same three divisions appear as in man. The ducts of two embryos of this period were graphieally reconstructed, taking the lateral borders of the wolffian ducts arbitrarily as straight lines. It appears from these reconstructions (figs. 2 and 3) that the area of direct contact of the wolffian and miillerian epithelia is considerably longer in the chick than in the human embryo. Due to the much greater diameter of the wolffian duct in the chick embryo, and also to the smaller diameter of the caudal portion of the miillerian duct, the relation of both appears slightly different on cross section, too, with the wolffian duct considerably exceeding the miillerian duct in size (figs. 18, 14). However, the essential features in the relation of the epithelia are the same here as in man, each level going through a stage of direct contact of both, followed by their separation first by basal membranes and later by increasing amounts of mesenchyme. The relations of the miillerian duct to the wolffian duct and the tubal ridge near the cloaca could not be clearly determined in the one sufficiently advanced specimen available (7 days, 17 hours). Further study of this area in a greater number of embryos will be necessary.

The appearance of normal stages indicates a great simillarity in the development of the miillerian ducts in the human and chick embryo. This supports the conclusion reached by comparison of spontaneous human malformations with experiments in chick embryos, namely that the mechanism of the early development of the miillerian ducts is the same in both species,

In both figures 2 and 3 the miillerian ducts appear distinctly longer, that is farther advanced, on the left side than on the right. The material at hand does not allow a definite statement to be made that this is the rule in the chick embryo. Should a large number of embryos show that this is the fact, a new contribution to the problem of lateral differences in the sex organs of birds would be given. It is hardly possible that a primary unequal distribution of primordial sex cells, as believed by some authors to be the cause of the asymmetry, can influence the growth rate of the miillerian ducts.


The present observations show that the relations of the growing wolffian and miillerian ducts are not as clear as it appears from descriptions in the textbooks. There can be no doubt that the growing caudal end of the miillerian duct lies within the basal membrane of the wolffian duct. This was described long ago by Nagel (1889) for the human, and by Balfour and Sedgwick (1879) for the chick embryo; but, as pointed out in the introduction, is not considered by recent authors. Our observations recall the old theory of splitting of the wolffian duct. The immediate contact of wolffian and miillerian epithelium is in favor of this conception, but the conditions are far from allowing the exclusion of the other version of, at least materially, independent growth of the mullerian duct. With the facts at hand, a decision is impossible. Development of the miillerian duct from cells of the wolffian duct, although suggested by the close proximity of both and the absence of separating basal membranes, does not explain the scarcity of transitional cell types. We must remember, however, that the amount of transitional stages present at a given moment depends on the speed of transition : the greater the speed, the smaller the number of cells caught during transformation.

When following Nagel in assuming that the miillerian duet, although closely attached to the wolffian duct, grows independently, we would postulate the very peculiar and, to the author’s knowledge, unique process of one epithelium growing for a long distance between another epithelium and the latter’s basal membrane. In an effort to obtain additional informaticn, the incidence of mitoses at the growing end of the miillerian duct and the adjacent portion of the wolffian duct was investigated, but failed to reveal indicative results. This is not surprising; the tip of the miillerian wedge, consisting of cells that may have split off from the wolffian duct, is so thin that its formation would not necessarily cause an increase in the number of mitoses beyond the limits of normal variation.

Regardless of which theory will eventually be proved to be correct, we are now in a much better position to understand the effect of experimental or spontaneous interruption of the wolffian duct upon the growth of the miillerian duct. The miillerian duct is absent in those levels showing absence of the wolffian duct not because it needs a stimulus of the latter to grow as was believed before (Gruenwald, ’37), but because it is topographically so closely related to the wolffian duct that it cannot develop alone. Assuming that it splits off from the latter, we can understand without difficulty its absence where there is no wolffian duct. Assuming, on the other hand, that it grows by itself between epithelium and basal membrane of the wolffian duct, we expect it to be caught in the blind end of the basal membrane if the wolffian duct fails to continue its growth. At such an abnormal end of both ducts far from the urogenital sinus the same thing happens as at their normal ends: the separation by basal membranes and mesenchyme continues until it reaches the ends and no area of contact of both epithelia remains. This can easily be seen in the embryos used previously for experimental destruction of a wolffian duct.

Malformations Resulting From The Fusion Of The Wolffian And Mullerian Ducts

The close developmental relations of the wolffian and miillerian ducts have considerable teratological importance. In the chick embryo experimental destruction of the caudal part of a wolffian duct results in aplasia of the kidney (because of absence of the ureteric bud) and of the miillerian duct to an extent equal to the defect of the wolffian duct. In man, the corresponding conditions exist as the most common combinations of spontaneous malformations of genital and urinary organs. This analogy indicates that the mutual relations of the developing urogenital organs in man are essentially the same as those shown by experiments in the chick, and also points to the explanation of these malformations as sequels of a spontaneous interruption in the growth of the wolffian duct (Gruenwald, 738). In the female the syndrome referred to consists of aplasia of one kidney, absence of an entire fallopian tube or its medial part, and uterus unicornis of the opposite side due to absence of half of the uterus on the defective side. The defect of tube and uterus in these cases is caused by the inability of the miillerian duct to grow in‘ the absence of the wolffian duct. (The fact that the miillerian epithelium is in its caudal parts probably replaced secondarily by epithelium of the urogenital sinus (Vilas, ’32; Koff, 33; Mever, ’34, ’37) does not influence these considerations. The sinus epithelium seems to follow closely the forms created by the retreating miillerian ducts.)

There is another malformation that can in all probability be explained by the close contact of the wolffian and miillerian ducts as described here; ectopic ureter opening into the vagina or uterus. Taking the structural independence of both embryonic ducts for granted, most authors were unable to find a reasonable explanation for this malformation. Gruber (’25), an authority in the field of malformations of the urinary organs, flatly stated that the details of the genesis of this malformation are unknown. Hepburn, in a discussion of Furniss’ paper on this subject (see Furniss, ’37), went as far as to suggest secondary anastomosis as an explanation of ureters opening into the uterovaginal canal because he could think of no other possibility. Meyer (’02) observed in a newborn female a ureter opening into a considerably dilated persistent wolffian duct, and later (’12) described a similar condition in a 70-mm. embryo. In both cases the dilated duct was closely attached to the uterovaginal canal, and Meyer considered perforation into the vagina a possible outcome. Meyer did not offer this as an explanation for all cases of vaginal openings of ureters, and it is, indeed, improbable that the ureter or wolffian duct should always perforate into the uterovaginal eanal at its end and that its enormous dilatation should in many cases disappear without a trace after perforation. Thom (’28) attempted to explain communications between ureter and vagina on the basis of Mijsberg’s (’24) embryological investigation of the development of the vagina. Mijsberg had found that the wolffian ducts participate in the formation of the vagina. This would explain how ureters remaining connected with the wolffian duct, can open into the lower portion of the vagina, but openings into the upper portion or the uterus would still remain unexplained. Moreover, Mijsberg’s findings were not confirmed by later investigators (Vilas, ’32; Koff, 83; Meyer, ’34, ’87).

Even when we take our present findings into account, we have to admit that there is not much chance for abnormal communications to result from the fusion of the wolffian and miillerian ducts. One of them always degenerates, so that such communications cannot appear in later periods. At the time when the miillerian duct reaches the pelvic region, the ureter has normally long been separated from the wolffian duct and cannot, therefore, be affected by persistent fusion.

Only for an abnormally developing ureter remaining connected with the wolffian duct, is there the possibility of becoming attached to the miillerian system by persistence of the fusion described here. Such a ureter may then open into the uterovaginal canal in the female, or into the prostatic utricle in the male. (Again, it is to be remembered that an eventual replacement of miillerian cells by those of the urogenital sinus does not change the relations.) Ureters opening into the vagina have been reported many times. Thom (’28) found in his extensive review of the literature 34 cases among 117 ectopic ureters in females, and many more have been reported since. Only three ureters draining into the uterus appear in Thom’s review, probably owing to the fact that very few ureters are connected at such a high level of the wolffian duct that they can become attached to the later uterus. As to ureters opening into the prostatic utricle, no reports are known to the author. It is, however, highly probable that this condition would be, and has been mistaken for a simple urethral opening of a ureter. Many of the ectopic ureters are double ones. This is not surprising because in the case of doubling the chances of abnormal topographic relations of one of the ureters are by far greater than in single ureters.

It is, of course, impossible to prove the correctness of this explanation unless suitable embryonic stages are discovered. However, to the author’s knowledge, no other explanation on a correct embryological basis can be given at the present time.


The present observations show that the miillerian duct is clearly separated from the wolffian duct only during the first phase of its development, that is the formation of the abdominal ostium. As soon as tube formation begins, a very close relationship between both ducts is established, so close, in fact, that the leading conception of independent growth of the miillerian duct can no longer be considered as definitely correct. Although it may not seem desirable to discredit one conception without substituting a better one, we have again to consider the abandoned theory of splitting of the wolffian duct along with that of materially independent growth of the millerian duct, and so far no decision can be made.

Our observations bring us closer to the explanation of the mechanism of the wolffian duct’s action as a ‘‘guide’’ of the miillerian duct. The latter cannot develop in absence of the wolffian duct because, according to one alternative, it grows inside of the latter’s basal membrane, or, according to the other conception, its cells are derived from those of the wolffian duct. Due to this dependence, spontaneous absence of a wolffian duct leads to a syndrome of defects comprising not only mesonephros and kidney but also the uterovaginal canal of the affected side.

Ureters remaining attached to the wolffian duct for an abnormally long period, may by means of the intimate contact of the wolffan and miillerian ducts become connected with the uterovaginal canal.


  1. In a large number of human and chick embryos, the caudal end of the growing miillerian duct was found to be very intimately connected with the wolffian duct. No basal membrane separates the epithelia and occasionally transitional cells were seen that could not be attributed with certainty to either of the two structures.
  2. Accordingly, the description of the miillerian duct as growing without contribution from the wolffian duct, cannot be taken for granted any longer. It is now to be considered as an alternative conception along with the older one which holds that the miillerian duct splits off from the wolffian duct. Thus far no decision ean be made.
  3. The close topographic relations of both ducts explain the experimentally proved dependence of the growing miillerian duct on the presence of the wolffian duct.
  4. In abnormal development, arrested growth of a wolffian duct causes not only agenesis of mesonephros and metanephros, but also absence of the miillerian duct in an area corresponding to the defect of the wolffian duct. This leads in the female to the relatively frequent association of uterus unicornis and defect of a fallopian tube with absence of one kidney.
  5. The relations described here make it possible to understand how an abnormal ureter may be connected with the uterovaginal canal; this may explain ectopic openings of ureters into the vagina or uterus.

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MeyeER, R. 1902 Einmiindung des linken Ureters in eine Uterovaginaleyste des Wolff ’schen Ganges, Ztschr. f. Geburtsh. u. Gyn., vol. 47, pp. 401-429.

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MissBere, W. A. 1924 Ueber die Entwicklung der Vagina, des Hymens und des Sinus urogenitalis beim Menschen. Ztschr. f. Anat. u. Entweklngsgesch., vol. 74, pp. 684-760.

NaceL, W. 1889 Ueber die Entwickelung des Urogenitalsystems des Menschen. Arch. f. mikr. Anat., vol. 34, pp. 269-384.

THom, B. 1928 MHarnleiter- und Nierenverdoppelung mit besonderer Beriicksichtigung der extravesikalen Harnleitermiindungen. Ztsehr. f. Urol., vol. 22, pp. 417-468.

Vinas, E. 1932 Ueber die Entwicklung der menschlichen Scheide. Ztschr. f. Anat. u. Entweklngsgesch., vol. 98, pp. 263-292.

Explanation of Figures

Plate 1

Sections showing the growing end of the miillerian duct in human embryos. M, miillerian duct; W, wolffian duct.

4 12-mm. embryo (section 15-4-6), azan stain.

5 13.5-mm. embryo (section 17-4-2), azan stain.

Same embryo (section 17~4—5), azan stain.

14 mm. embryo (section 18-3-2), Gomori’s silver impregnation. 15.5-mm. embryo (section 22~5-7, right side), azan stain. Same embryo (section 23-3-4, left side), azan stain.

Plate 2

Sections showing the growing end of the miillerian duct in human and chick embryos. M, miillerian duct; T, tubal ridge (thickened coelomi¢ epithelium) ; U, urogenital sinus; W, wolffian duet.

10 18-mm. human embryo (section 22-5-7),

11 25-mm. human embryo (section 52-5-6),

12 30-mm. human embryo (section 81-6-3), tact of wolffian and miillerian epithelium.

13 4day 5 hour chick embryo (section 13-3-5), azan stain. 14 Sday Ghour chick embryo (section 13-2-2), Gomori’s silver impregnation. The reconstruction shown in figure 2 is based on the same embryo.

15 Sday 12 hour chick embryo (section 15-4-1), Gomori’s silver impregnation.

The mesonephrie ridge is cut longitudinally.