IV 



COELENTEEATA 



77 



The egg of Urticina when discharged is surrounded by a gelatinous 

 mass, through which presumably the fertilizing spermatozoon has to 

 penetrate. After fertilization this gelatinous mass hardens into a firm 

 capsule beset with spines, and within the shelter of this capsule the 

 early stages of development are passed through. The egg is composed 

 of a thin rind of relatively clear cytoplasm and an internal zone of 

 cytoplasm loaded with large spheres of yolk. The kernel of the egg 

 consists of a mass of material with sparse yolk spheres but with many 

 granules, which appears to be reserve food material not cytoplasm. 



The egg is thus of the type called centrolecithal in Chapter L, 

 and its segmentation is a matter of considerable interest. The zygotic 

 nucleus is situated in the outer 

 clear layer of cytoplasm, and there 

 it undergoes its first division. 

 The daughter nuclei migrate into 

 the deeper layer of yolky cyto- 

 plasm and here undergo repeated 

 division until sixteen nuclei 

 have been formed, which are 

 distributed around the periphery 

 of the egg in the yolky layer. 

 Then and then only the cytoplasm 

 begins to be divided into blasto- 

 meres, of which consequently 

 sixteen are produced at once. 

 At first (Fig. 57) the blastomeres 

 are separated from one another FlG- 5 7._ An e gg of Urtidna 



only at their Outer ends, but dividing into sixteen blastomeres. (After 



they soon become sharply de- AppeDof.) 



fined Over their Whole Surfaces ; w > blastomeres ; c, core of nutritive material in 



,11 i j the centre of the egg ; i.z, inner zone of yolky cyto- 



nevertheless, even when so de- plasm . . Z) outer zone of c i ear cyt opiasm. 

 fined, their inner ends are 



embedded in the mass of reserve material which forms the innermost 

 core of the egg. 



As segmentation proceeds a blastula is eventually formed, whose 

 wall consists of a single layer of small cells, but in whose cavity there 

 still remains the mass of " reserve-material " which formed the kernel 

 of the unsegmented egg (Fig. 58). 



The blastula is at first ellipsoidal, but one pole becomes flattened 

 and in the centre of this pole an invagination takes place. The 

 manner in which this occurs is of great interest. The borders of the 

 patch which is to be invaginated bend in first, so that for a brief 

 period its central part projects like a knob. As the process of 

 invagination proceeds the centre is also carried downwards and 

 inwards, and thus a two -layered organism i.e. a gastrula, is 

 produced. Occasionally the reserve material in the blastocoele persists 

 in considerable quantity ; it adheres to the centre of the area which 

 is normally invaginated and thus impedes the process of invagination. 



