1 88 GENERAL PRINCIPLES OF ZOOLOGY. 



Invagination. The gastrula is formed from the blastula 

 by imagination (Fig. 101, A). The result is the same as 

 when by pressure of the finger upon a hollow india-rubber 

 ball one side is pressed in against the other; the layer of 

 vegetal cells gradually sinks in and becomes surrounded 

 by the cells of the animal pole (Fig. 101, B}. Thus there 

 arises in the egg, in addition to the cleavage cavity, a new 

 cavity, the rudiment of the lumen of the digestive tract ; 

 this increases and finally entirely obliterates the cleavage 

 cavity, so that the invaginated part of the blastoderm, the 

 entoblast, becomes pressed against the part which remains 

 external, the ectoblast. 



Modified Modes of Gastrulation. In the case of eggs 

 with much food-yolk, the relation of the structure and of 

 the mode of formation of the gastrula is more difficult to 

 understand. Here, however, it is sufficient to mention the 

 fact that the gastrula stage has fortunately been discovered 

 for almost all eggs with a great quantity of food-yolk, and 

 that the yolk-material finds lodgment principally in the 

 entoblastic cells. 



Epiblast and Hypoblast. For outer and inner germ- 

 layer the terms epiblast and hypoblast, upper and lower germ- 

 layer, have been much used ; these names actually apply best 

 only to those eggs with discoidal cleavage. In the bird's 

 egg, for example, the two germ-layers form over the unseg- 

 mented yolk, from which they become separated by the 

 gastrular cavity ; thus then the external germ-layer actually 

 lies above, the internal below. Other terms for the two 

 germ-layers are ectoderm and cutoderm. These names were 

 originally used for the body-layers of mature animals, the 

 ccelenterates, and were only later transferred to embryology. 

 In this book they will be used only in their original 

 signification for cell-layers which have already undergone 

 organological and histological differentiation, since for em- 

 bryonic cell-layers the names entoblast and ectoblast are 

 more suitable. 



Delamination. In regard to the mode of development 

 of the gastrula many controversies have arisen which are 



