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HANDBOOK OF INVERTEBRATE ZOOLOGY. 



which has been developed from the cells formed at the 

 nutritive pole, and pushed inwards to form the lining wall 

 of the digestive cavity, and the ectoderin, which is formed 

 from the formative pole of the egg, and is to give rise to 

 the outer wall of the body. The segmentation cavity ({), 

 is no longer spherical, since the ingrowth of the digestive 

 cavity encroaches upon it. The opacity of the egg of 

 Arbacia prevents accurate study of its internal structure, 

 but in Strongylocentrotus careful examination with high 

 powers will show that the inner ends of the endoderm 

 cells are separating off and forming stellate amoeba-like 

 cells, which are free in the segmentation cavity. These 

 are the mesoderm cells, which after a time become ar- 

 ranged in a layer around the segmen- 

 tation cavity on the inner ends of 

 both ectoderm and endoderm cells. 

 The outer surface of the body now 

 becomes covered with fine cilia, and 

 the embryo escapes from the egg- 

 shell, and swims at the surface of the 

 water. 



Fig. 59. — Side view of the larva shortly after 

 its escape from the egg. (Drawn from nature 

 Fig. 59. by W. K. Brooks.) 



During the second or third day, the embryo elongates 

 in a line nearly at right angles to the principal axis, and 

 at the same time becomes nearly triangular when seen in 

 side view (Fig. 59). The angles are short and rounded, 

 and one of them (a), is at what may now be called the an- 

 terior end of the body, another (b), at the posterior end, 

 and a third near the middle of what will be called the 

 ventral surface. The longest side {a,b), is nearly straight, 



