52 INVERTEBRATE MORPHOLOGY. 



Eight segmentation-cells are thus formed which remain in 

 contact with each other and enclose a small cavity, the seg- 

 mentation-cavity or blustocoel. The further division of the 

 cells (Fig. 24, C) results in the formation of an oval or spheri- 

 cal organism (Fig. 24, D] which may be compared to Volvox, 

 consisting of a single layer of cells enclosing a more or less 

 voluminous blastocoel. This embryonic stage is known as 

 the blostula. In its simplest form it shows no special differ- 

 entiation into tissues, its cells being uniformly ciliated, and 



FIG. 24 DIAGRAMS ILLUSTRATING THE SEGMENTATION OP THE OVUM. 



A, four-celled stage. 



B, eight-celled stage of a telolecithal ovum. 



C, sixteen-celled stage. 



D, blastula. 



The arrows iudicate the mode of division. 



the organism free-swimming, moving through the water with 

 a rotatory movement about a definite axis, one and the same 

 end of which is always anterior. In many blastulas, however, 

 especially in those which for one reason or another are not 

 free-swimming, an early differentiation of the cells takes place, 

 especially at the extremity which is posterior in the free- 

 swimming forms or which corresponds to that pole in the 

 non-motile embryos. These posterior cells are usually some- 

 what larger than those at the anterior pole, and if much food- 

 yolk is present in the embryo it is especially concentrated in 



