DEVELOPMENT 



193 



fishes, or birds, the process of segmentation may appear to differ 

 greatly from that of the scheme of Fig. 26, i to 4 : Nevertheless, 

 the typical mode is that of the scheme and the latter can be 

 recognized, even in greatly distorted forms. 



Typically, then, the process of segmentation continues until over 

 1,000 small blastomeres have been formed. These then become 

 arranged in a particular way. 



The Blastula. This is a little hollow ball, the wall of which 

 is formed by a single layer of cells, which are often ciliated so that 

 the embryo is mobile at this stage. 



Ectode 



Arche-nteroTL 



EctodberTn, 



MesoderTn 'EndjoderrrL 



7 



Fig. 27. — Blastular and Gastrular Stages. 



I, The blastula ; 2, blastula beginning to invaginate ; 3, invagination complete, the gastrula 

 stage ; 4, gastrula in a yolky egg ; 5, complete gastrula ; 6, free-living gastrula larva of a 

 worm ; 7, formation of the germ-layers in the hen's egg. 



The process of gastrulation. At some part of the blastular wall, 

 say at x in Fig. 27, i, the cells divide more rapidly than elsewhere. 

 The result must be that this part of the wall is either pushed 

 outwards or inwards. Normally it is pushed inwards, so that 

 the blastula is " dimpled." The process of '' invagination " 

 continues until a double- w^alled embryo is formed. This is the 

 gastrula (Fig. 27, 3). The space between the two walls is the 

 " segmentation " cavity and the inner cavity is the " archentron," 

 which communicates with the outside by the development of the 

 original dimple, or " blastopore." 



Thus an embryogenic stage is attained which is called the 

 " gastrula " and this occurs very often and in all the phyla of 

 animals which have the three formative layers, endoderm, meso- 

 derm and ectoderm. It may be difficult to recognize because 



o 



