EARLY DEVELOPMENT 



603 



after being laid, and they then undergo segmentation into a 

 number of equal, or nearly equal, segments or " blastomeres." 

 These arrange themselves in the form of a hollow sphere or 

 " blastula," the cavity of which is called the " blastocoel " and 

 afterwards becomes the primary body-cavity of the larva. This 

 cavity contains an albu- 

 minous fluid, at the 

 expense of which de- 

 velopment appears to 

 be carried on (Fig. 

 282, B). The cells 

 forming the blastula 

 acquire cilia, and the 

 embryo begins to rotate 

 within the egg -mem- 

 brane, which it soon 

 bursts, and, rising to 

 the surface of the sea, 

 begins its larval life. 

 The blastula is there- 

 fore the first well- 

 marked larval stage, 

 and it is found in a 

 more or less recognis- 

 able form in life -his- 

 tories of members of 

 every large group 

 the animal 

 Only in the case of 

 Echinodermata and of 

 forms still lower in the 

 scale, however, does it appear as a larval stage. The free- 

 swimming blastula stage is reached in from twelve to twenty-four 

 hours. Soon the spherical form of the blastula is lost ; one side 

 becomes flattened and thickened, owing to a multiplication of cells, 

 so that they become taller and narrower in shape. Shortly after- 

 wards this thickened plate becomes buckled inwards, encroaching 

 on the cavity of the blastocoel. The larva has now reached 

 the second stage of its development ; it has become a " gastrula " 

 (Fig. 282, C). The plate of thickened cells has become con- 



in 



kingdom. 



Fig. 281. Hemiaster philippi. Enlarged view of a 

 single petal, showing the embryos in situ. (From 

 Wyville Thomson.) The whole animal is shown in 

 Fig. 250, p. 555. 



