IX 



PHYLUM ECHINODERMATA 



381 



placed on the ventral surface. When the ova have been dis- 

 charged and impregnated, they adhere by means of a viscid 

 investment to the surface (rock or stone) on which they are laid, 

 and go through all the stages of their development in this position, 

 never passing through a free pelagic stage. The eggs are about 

 half a millimetre in diameter, and of a spherical shape. Each con- 

 sists of a perfectly opaque central mass of yellow or orange yolk, 

 and of a glassy layer enclosing this. After fertilisation the process 

 of segmentation begins by the division of the ovum into two blasto- 

 meres almost equal in size, but one, which may be termed cell 

 I., slightly smaller than the other (cell II.). Both I. and II. soon 

 afterwards divide, I. somewhat earlier than II. The resulting 



FIG. 318. Early stages in the development of a Starfish (Asterina gibbosa). A, eight-celled 

 stage ; B, stage of about thirty-two cells seen in section ; C, gastrula stage ; D, section of 

 early gastrula ; E, section of later gastrula. arch, archenteron ; blastoc. blastoccele ; blp. 

 blastopore ; ect. ectoderm ; end. endoderm. (Modified after Ludwig.) 



four cells again divide, leading to the formation of an eight-celled 

 stage (Fig. 318, A), in which the four cells derived from I. form 

 an incomplete ring not closed below, and the four derived from 

 II. form an incomplete ring open above. 



The eight cells then divide by meridional fissures into 

 sixteen, and a further division results in the formation of thirty- 

 two. The thirty-two cells become arranged in such a way as to 

 enclose a central cavity which had been present in the four-celled 

 stage : this stage (B) is the blastula ; the cavity is the segmenta- 

 tion-cavity or blastoccele. The number of cells in the wall of this 

 cavity increases by further divisions, and the whole surface becomes 

 covered with vibratile cilia. A process of imagination then 

 follows, one side of the blastula being pushed inwards to form 

 a double-walled cup or gastrula (C) opening on the exterior by 



