160 EMBRYOLOGY 



Rhabdocoeles in turn resembles that of tlie Polyclads ; the 

 Rhabdocoeles are, however, like the Triclads in the produc- 

 tion of yolk cells. 



I. POLYCLADIDA. 



A. Direct Development. 



The development of the Polyclads .has been described, 

 chiefly in the works of GtOETTE (No. 3), Hallez (No. 6), 

 Selenka (No. 20), and Lang (No. 13). 



The eggs, united by means of a slimy secretion, are usually 

 laid in the form of a unilaminar plate, in which they lie 

 more or less regularly side by side. In the Eurijleptidce they 

 are attached to some support by means of a stalk (Selenka, 

 Lang). Ordinarily each egg is surrounded by a thin shell, 

 which in some cases (Psetidoceridsg) is provided with an 

 operculum. Fertilization, which sometimes takes place after 

 oviposition, is usually preceded by the formation of the two 

 polar globules. These do not sepai'ate at once from the egg, 

 but remain united to it by means of yolk-substance. The 

 spermatozoon then passes between them in penetrating into 

 the egg. Such is the process in Thysanozoön, according to 

 Selenka's obsei'vations. Since only one spermatozoon is 

 bestowed upon each egg, the act of fertilization in this 

 instance seems always to be accomplished with great cer- 

 tainty. 



Cleavage is unequal. Even the first two blastomeres are 

 of different sizes. Each of them divides into two, and 

 these four blastomeres also differ in size. Owing to their 

 differences in position and size, the various regions of the 

 body of the embryo, it is said, are already indicated. At 

 first the two smallest blastomeres lie crosswise over the 

 larger ones (Fig. 75 A). They indicate the upper, aboral 

 pole, a conclusion which is confirmed by the polar globules, 

 since these lie above them, whereas the two large blasto- 

 meres correspond to the lower, oral pole. Furthermore it 

 is shown that even thus early the anterior end of the animal 

 is indicated by the smaller of the two large blastomeres, 

 the posterior end by the larger one, and that the two 

 smallest blastomei^es correspond to its sides. 



