DEVELOPMENT. 



185 



How are changes in the developing ovum commonly studied? 



In the development of the egg of the common fowl. This is 

 on account of the accuracy with which the time of development 

 may be watched, and the convenience to the observer of such sim- 

 ple growth by incubation compared with uterine growth. The pro- 

 cesses of development are not materially different. 



What changes occur in the blastoderm? 



There appears at one point an opaque streak (Fig. 57), which is 



FIG. 57. 



Diagram of the Area Germinativa, showing the primitive trace and area pellucida. 



found to be due to the proliferation of the cells of the blastoderm. 

 This is the primitive trace, and it grows in length and breadth. 



What changes occur in the arrangement of the cells of the blasto- 

 derm at this time? 



They separate into three layers, the epiblast, mesoblast, and hypo- 

 blast (Fig. 58) ; and along the axis of the primitive trace a groove 

 is formed which is destined to become the cerebro-spinal axis. This 

 is the primitive groove. 



What changes occur in the zona pellucida at the entrance of the 

 ovum into the uterus? 



The surface of the ovum becomes shaggy with the appearance of 



