XI 



POLYZOA 



399 



marked off from both animal and vegetable cells, they are rounded 

 and have unusually large nuclei which are surrounded by clear areas. 

 The blastula now slightly elongates so that it is bilaterally 

 symmetrical, and its vegetative half becomes flattened, and these 

 two peculiar cells are then situated right and left of the plane of 

 symmetry. The cells of the vegetative half of the egg become then 

 invaginated, and the gastrula stage is attained. The two large cells 

 mentioned above are situated at the posterior end of the blasto- 

 pore, but they now pass into the blastocoele and are the mother 

 cells of the mesoderm. The blastopore is at first slit -like but 



end 



stem 



mes 



FIG. 318. Early stages in the development of the egg of Pedicellina echinata. 

 ( After Hatschek.) 



A, flattened blastula. B, gastrula. C, optical section of later stage nearly sagittal showing the 

 mother cell of the mesoderm of the right side, the endodermic sac, and the stomodaeal invagination. 

 D, view of stage in which the blastopore is closing seen from below. E, frontal section of the same 

 stage ; Up, blastopore ; end, endodermic sac ; mes, mother cells of the mesoderm. 



gradually becomes restricted in length, becoming closed from behind 

 forwards. The ectoderm, endoderm, and mesoderm are thus definitely 

 differentiated from one another. 



The ectoderm flattens out except at the aboral pole. Here an 

 apical plate of cylindrical cells is formed. This was termed the cement- 

 organ by Hatschek, under a mistaken idea that it was the plate by 

 which the larva fixed itself. This plate develops cilia but it soon 

 begins to be invaginated. When the complete larval stage has been 

 attained the walls of the invagination are still ciliated, but the cells 

 forming the floor multiply so as to form a mass of small rounded 

 cells several layers thick; these are ganglion cells as the develop- 

 ment of the neuropile around them shows. 



