308 



INVERTEBEATA 



CHAP. 



divide, but acquired the long powerful cilia characteristic of a 

 trochoblast. 



The macromeres of the 8-cell stage, i.e. lA, IB, 10, and ID, 

 when isolated, usually either died or formed masses of cells which 

 disintegrated ; but in some few cases larvae were obtained. In all 

 cases a second quartette cell was formed, which divided as it 

 normally does, into a lozenge-shaped group of four cells ; then a third 



Fia. 243. — Illustrating the results obtained by separating the blastomeres of the 

 developing egg of Patella coerulm. (After Wilson.) 



A, under view. B, lateral view of tlie larva, resulting from the development of a single isolated 

 micromere of the first quartette. The four primary trochoblasts are shaded. The other cells bearing 

 cilia in B are apical cells, C, lateral view of the larva, resulting from the development of one of the 

 four macromeres of the S-eell stage. Two secondary trochoblasts are seen, and two feebly ciliated cells, 

 which are probably a portion of the mid-ventral ciliated groove. The endoderm cells are shaded. D, 

 lateral view of the larva, resulting from the development of one of the micromeres of the second 

 quartatte. Two secondary trochoblasts are seen, and two feebly ciliated cells, probably a portion of 

 the mid-ventral ciliated groove. The interior of the larva contains loose cells representing the so- 

 called mesectoderiu. 



quartette cell was formed which divided into two, and a cell belonging 

 to the fourth quartette was also produced. 



When a larva was obtained from one of these isolated macro- 

 meres the mass of cells belonging to the second and third quartettes 

 were seen to have undergone such displacement as to form a complete 

 ectodermic covering for the macromeres, and gastrulation had 

 occurred. At the one end of the embryo were two cells bearing 



