ZOOLOGY AND BOTANY, MIGEOSCOPY, ETG. 109 



and the difference in the fate of the deuterogenetic centre, are of funda- 

 mental importance in an attempt to understand the relationship of the 

 chief phyla. The experimental and anatomical facts point to the con- 

 clusion that the main axis of the Vertebrata is at right angles to the 

 plane of the blastopore or ccelenterate mouth. 



Origin of Sex-cords and Spermatog'onia in Male Chick.*— Charles 

 H. Swift finds that the true sex-cords or seminiferous cords originate 

 from the germinal epithelium during the sixth and seventh days of 

 development, and are the result of localized activity of the epithelium. 

 Xearly all the primordial germ-cells present in the germinal epithelium 

 are carried down into the seminiferous cords, but they play only a 

 passive role, for at this time they show no evidences of cell-division. 

 The sex-cords remain attached to the germinal epithelium for only a 

 short time, and continue to grow, after formation of the albuginea, as a 

 result of division of the peritoneal cells. 



At the end of the seventh day of development the sex of the 

 individual can be easily told, for in the male the gonads are of nearly 

 equal size, while in the female the left gonad is much the larger. In 

 the male the germinal epithelium remains thin after the formation of 

 the sex-cords and contains very few primordial germ-cells, while in the 

 female the epithelium of the left gonad or ovary continues to be thick 

 and contains many primordial germ-cells. During the eighth and ninth 

 days of development the gonads increase slowly in size and the thin 

 sex-cords make up most of the volume of the testes. They are separated 

 from one another by a thin layer of stroma, and have a definite orienta- 

 tion from germinal epithelium obliquely down towards the Wolffian 

 body. When the embryo is eleven days old the stroma begins to 

 increase in quantity and the seminiferous cords commence to meander 

 and to anastomose with each other. 



Up to the thirteenth day the primordial germ-cells in the sex-cords 

 do not divide. At this date they divide actively for four days. The 

 primordial germ-cells give rise to spermatogonia, which differ from 

 them in having a mitochondrial crescent (comparable to the " yolk- 

 nucleus " in oocytes and young oogonia). The interstitial cells appear 

 in the stroma on the thirteenth day, but reach their greatest development 

 during the seventeenth day. They are simply differentiated stroma 

 cells, since they do not divide and transitional forms can be seen. 



Cavities begin to appear in the network of seminiferous cords during 

 the twentieth day, arising by liquefaction of axial cells. At this date 

 the spermatogonia are found against the basement membrane, with the 

 nucleus towards the central axis of the cord, afid the mitochondrial 

 crescent near the basement membrane. They probably reach this 

 position by amoeboid migration. The elongated cells between the 

 spermatogonia are derived from the peritoneal cells of the seminiferous 

 cords. The primordial germ-cells give rise to the spermatogonia, and 

 the coelomic cells of the germinal epithelium produce the supporting 

 <;ells of the seminiferous tubule. 



• Amer. Journ. Anat., xx. (1916; pp. 375-410 (6 figs.). 



