DEVELOPMENT OF BIEDS AND MAMMALS 241 



canal and the endoderm are placed in communication by an 

 undifferentiated mass of multiplying cells which runs round 

 the posterior end of the notochord. 



The shutting in of the neural canal posteriorly is accom- 

 panied by a folding of the embryo immediately behind : this is 

 the tail fold ; and the folding in front and behind the embryo 



Optic ves. Forebr. Mes. End. Ect. 



hnocoel 



Aud. invag. 



Foregut 

 Splanchnocoel 



Splanchnocoel 



Aorta 

 Notochord 



Prone P*Kyocod 



FIG. 123. Three transverse sections of second-day fowl, the first 

 through the optic cups, the second through the auditory 

 invagination of the ectoderm, and the third through the 

 body behind the head. 



gradually extends, with the result that the embryo is raised 

 and defined from the blastoderm. The growth after the 

 formation of the tail takes place by a general expansion of the 

 embryo, and in length by the continued growth of the tail 

 fold that is to say, of the region occupied by the dorsal lip 

 of the blastopore. The ventral part of the blastopore by the 

 formation of the tail fold is bent under the embryo, and 

 from it later the proctodeum is developed. 



While these changes have been proceeding the ectoderm 

 and the endoderm have made immense progress in investing 

 the yolk. The blastoderm has extended over more than half 



