98 Embryonic Development of Ovary and Testis of Mammals 



former to the latter is noted in the nearer approach of the peritoneal 

 invaginations to the capsules of Bowman. They become less crowded 

 and the genital ridge decreases in height. 



1.5 and 1.6 cm. Embryos. — The sex gland increases in volume to such 

 an extent that in the 1.6 cm. stage it appears circular in cross-section. It 

 is attached to the mesonephros by its mesentery which is now much nar- 

 rower than the sex gland. The latter appears to have been constricted 

 off from the surface of the mesonephros by lateral furrows. This, how- 

 ever, is not the case, because measurements show the mesentery to be as 

 broad as the base of the sex gland of the 1.4 cm. stage. The constriction 

 is apparent, not real. In reality the centrifugal growth of the sex 

 cords has caused the sex gland to expand on all sides until it is now 

 cylindrical in shape, instead of appearing as a slight elevation above 

 the surface of the mesonephros as in preceding stages. 



The sex cords are becoming longer and more contorted. Together 

 with the growth in extent they become more clearly defined. The in- 

 crease in the surface of the peritoneum caused by the expansion of 

 the organ has not been accompanied by the formation of new cords, 

 hence space is left in which many sex cords become arranged parallel 

 to and immediately beneath it. 



There appears the beginning of a most important process by which 

 the sex cords become separated from the peritoneum through the de- 

 velopment of the albuginea (Plate III, Pig. 8). The nuclei of the cords 

 at their points of attachment to the peripheral cell layer (peritoneum) 

 begin to elongate and in many cases to assume the appearance of con- 

 nective tissue nuclei, while their cytoplasm is drawn out into slender 

 strands that stretch across the necks of the sex cords. At the same time, 

 a basement membrane is forming beneath the peritoneum dividing it 

 from the elongated cells just described. 



The mesentery is composed of cells derived from the dorsal and 

 ventral mesentery fundaments, their rather irregular arrangement 

 being disturbed by the ingrowth of a number of blood-vessels. 



At this stage appears the peritoneal invagination that forms the 

 Miillerian duct. It arises in the ventral part of a plate of thickened 

 epithelium which forms the anterior end of the genital ridge. Ex- 

 actly similar invaginations forming the most anterior rete tubules 

 arise in the same epithelial plate immediately dorsal to this rudiment 

 of the Miillerian duct. Prom the foregoing it seems fair to assume 

 that the Miillerian duct is homodynamous with the rete tubules and 

 sex cords. 



1.7 cm. Emhryo. — In this stage the sex cords become completely 



