CHAPTER XIII 



ORGANS FROM THE ENDODERM 



We may now turn again to the history of the development 

 of the normal embryo. 



The Closure of the Blastopore, and the Formation 

 OF the Neurenteric Canal 



During the last stages of the closure of the blastopore its 

 lateral lips rapidly approach each other, and it then becomes 

 an elliptical and later a slit-like opening (Fig. 23). The pos- 

 terior edge of the blastopore also groAvs forward for a short 

 distance, and as a result a pocket-like continuation of the 

 archenteron is formed (Fig. 37, A). The depth of this pocket 

 corresponds to the extent of the forward growth of the poste- 

 rior edge or ventral lip of the blastopore. If the embryo be 

 examined in the region over which the posterior lip of the hlasto- 

 jyore has advanced, there will be found at first nothing on the 

 surface to mark the region closed over. Some observers have 

 described faint traces of a groove in this region, but such 

 appearances are probably exceptional. Later, however, when 

 the outlines of the medullary folds have appeared, a distinct 

 longitudinal groove appears in this region running posteriorly 

 from the small blastopore (Fig. 23, B). At the ventral end 

 of the groove a distinct depression or pit is soon formed 

 (Fig. 37), which marks the beginning of the anus. It lies 

 at a point opposite to the bottom of the posterior pocket of 

 the archenteron, and corresponds therefore approximately to 

 the reo-ion at which the first trace of the ventral lip of the 

 blastopore was found. 



As the medullary folds close in to form the nervous system, 

 the blastopore is overarched by their posterior ends. The folds 



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