218 



EMBRYOLOGY 



subsequently prove to be mesenchymatous migratory cells 

 (Fig. 102 B and C), like those which arise in the develop- 

 ment of the type of Desor. 



After the blastula becomes covered with cilia, has assumed 

 its characteristic shape, and has acquired a large flagellum 

 at its apex (Fig. 102), it may break through the egg- mem- 

 brane to swarm about at large. More often, however, the 

 larva reaches the outside world only after invagination has 

 taken place, i.e. as a gastrula. This is accomplished by the 

 symmetrical invagination of the already-formed entoderm 



Fig. 102. — A to C, blastula, gastrula, and pilidium of Li'iieiis lactcus (after Metsch- 

 nikoff) ; C, combination of two of Metschnikofk's figures; s, ectodermiil in- 

 vaginations, which subsequently grow around the intestine as the prostomial and 

 metastomial discs. 



(Fig. 102 5). The blastopore is circular, and the entire larva 

 presents a radial form. This 'is soon changed, however, for 

 the blastopore elongates somewhat and becomes oval, while 

 the archenteron bends to one side, and its blind end groAvs 

 more and more toward one wall (Fig. 102 B). In this way 

 the form of the lai'va becomes bilaterally symmetrical. The 

 larva assumes its permanent shape — i.e., the one which its 

 discoverer, JoH. Müller, designated as pilidium — by the 



