PHYLUM PLATYHELMINTHES 



269 



from the megaroere with the corresponding capital ; those of the 

 second quartette are 2a, 26, 2c, and Id ; and so on with the others. 

 From the way in which the cells of the quartettes are given off, 

 the type of segmentation here exemplified, which prevails also in 

 the Annulata (Section X) and most of the Mollusca (Section XII), 

 is known as the spiral type. The cells of the first three quartettes 

 are all composed of cells (micromeres) smaller than those from 

 which they are derived. The formation of the third quartette 

 results in a stage (Fig. 219) in which the embryo consists 

 of thirty-two cells in all, since divisions take place in the 

 cells of the first and second quartettes. In later stages the 

 micromeres increase greatly by further division and extend as 

 a cap, the ectoderm, composed of a single layer of small cells 

 over all the upper part (animal) pole of the embryo ; then 

 spread further 



/st (juarteit? 



2nd 



quartette 



.1st quartette 

 5A 



2nd 



as a thin layer 

 over the entire 

 surface till 

 only a slit- 

 like blastopore 

 is left on the 

 ventral side : 

 finally the 

 blastopore 

 closes and the 

 ectoderm 

 forms a com- 

 plete invest- 

 ment. 



Meanwhile 

 the other em- 

 bryonic layers 

 have been es- 

 tablished. It 



is one of the cells of the fourth quartette that is alone or mainly 

 responsible for the origin of the endoderm and a great part of the 

 mesoderm. The four original megameres, A, B, C, and D, now 

 reduced to quite small cells (Fig. 220, mac), after the separating off 

 of the fourth quartette take no further part in development, and 

 are ultimately absorbed. Of the four cells of the fourth quartette, 

 which are comparatively large, three become broken up into masses 

 of yolk material to be subsequently absorbed as food ; but they may 

 (Discocoelis) first give off cells which contribute to the formation 

 of the endoderm. The fourth (designated 4:d) gives rise to the 

 whole of the endoderm or (in Discocoelis) the whole with the 

 exception of the portion derived from the other three cells of the 

 fourth quartette and a considerable part of the mesoderm. 4d 



2nd 

 quartette 3D 



FIG. 219. Developing egg of Planocera : thirty-two cell stage, 

 seen from the lower (vegetative) pole. As the four original 

 megameres, A D, have now each given off three cells in succession, 

 their designations are now 34 3D. (From MacBride, after Surface.) 



