66o 



TEXT-BOOK OF PHYSIOLOGY. 



DEVELOPMENT OF FETAL ACCESSORY STRUCTURES. 



Segmentation of the Ovum. Shortly after the formation of the 

 parent cell, segmentation of the nucleus and cytoplasm takes place 

 in accordance with karyokinetic methods. The two new cells thus 

 formed undergo a similar division into four, the four into eight, the 

 eight into sixteen, and so on until the space within the zona pellucida 

 is completely filled with a large number of small cells, each possessing 

 the characteristic cell structures. The peripheral cells then arrange 

 themselves in the form of a membrane, and as they are, at the same 

 time, subjected to mutual pressure they assume a polyhedral shape, 

 and give to the membrane a mosaic appearance (Fig. 326). The 



central cells then 

 undergo liquefac- 

 tion. At some 

 point on the inner 

 surface of the 

 membrane, cells ac- 

 cumulate which by 

 their division and 

 multiplication form 



FIG. 326. PRIMITIVE TRACE OF THE EMBRYO, a. Primi- 

 tive trace, b. Area pellucida. c. Area opaca. d, 

 Blastodermic cells, e. Villi beginning to appear on 

 the surface of the zona 



a second mem- 

 brane. The two 

 together are known 

 as the external and 

 internal blasto- 

 dermic membranes. 

 Germinal Area. 

 At about this 

 period there is an 

 accumulation o f 

 cells at a certain 

 spot in the sub- 

 stance of the blastodermic membranes which marks the position of 

 the future embryo. This spot, at first circular, soon becomes 

 elongated. A slight indentation now develops into what is known as 

 the primitive trace. Around this area there is a clear space, the 

 area pellucida, which is in turn surrounded by a darker region, the 

 area opaca. The primitive trace soon disappears and the area 

 pellucida becomes guitar-shaped. A second groove, the medullary 

 groove, is now formed, which develops from before backward and 

 becomes the neural medullary canal. 



Blastodermic Membranes. The embryo, at this period, con- 

 sists of three layers viz., the external and the internal blastodermic 

 membranes and a middle membrane formed by a genesis of cells 



