520 EMBRYOLOGY OF. THE CHICK 



In the female, the epithelium increases in thickness to an enormous 

 extent. The primitive ova multiply, and the products of this multiplica- 

 tion, accompanied by some of the epithelial cells, sink into the deeper 

 stroma of the connective tissue, and thus form ovarial or medullary 

 cords, each such cord containing a number of ova. The cords then break 

 up, each egg becoming surrounded by a layer of epithelial cells, the 

 whole forming a Graafian follicle. The follicle cells supply the nourish- 

 ment to the egg lying within. 



This whole growth takes place only on the left side of the chick, as 

 the right ovary is not functional. 



In the male, the beginnings of the gonad formation are similar to 

 that of the female, but instead of the cords breaking up into separate 

 follicles, each cord, develops a lumen which becomes converted into the 

 seminiferous tubule. One can, however, see in the walls of these tubules 

 both types of cells that were seen in the Graafian follicle. Indeed, there 

 is found a third type of cell called Sertoli's cell, which is supposed to 

 act as a sort of nutritive or nurse cell to the developing sperm. 



THE ADRENAL BODIES 



While these bodies lie closely attached to the kidney, they have not 

 developed as a part of the urinary system. 



It is important to know that the adrenal organs, which are among 

 the prominent ductless glands now studied in the schools, arise from two 

 separate and distinct origins : 



First, by a proliferation of peritoneum, and second, by a prolifera- 

 tion of the sympathetic ganglion cells. It is the portion arising from the 

 p'eritoneum which connects with the mesonephros. 



The peritoneal proliferations begin as cords or strands of cells along 

 the dorsal aorta. These then connect with the renal vesicles of the 

 mesonephros. Later, the sympathetic proliferations extend within the 

 peritoneal cords, so that the peritoneal cords now become the cortex and 

 the sympathetic portions become the medulla of the adult adrenal 

 glands. 



THE. CIRCULATORY SYSTEM 



At this point it is well for the student not only to realize, but to 

 appreciate the great number of experiments necessary to demonstrate 

 biological facts, as well as to understand the great number of possible 

 errors and objections which men may bring forth to oppose the inter- 

 pretation of these facts after the facts themselves have been demon- 

 strated. 



Suppose the question be raised as to whether the first beating of 

 the heart of an embryo is muscular or nervous in type. What experi- 

 ments, for example, would be necessary to answer such a question sat- 

 isfactorilv? 



