146 



GENERAL PRINCIPLES OF ZOOLOGY 



narrower mouth. The cavity of the cup (the primitive digestive tract or 

 arclienlerori) is the beginning of the most important part of the digestive 

 system ; the opening is the primitive mouth or blastopore. Of the two layers 

 of cells forming the wall of the cup and uniting at the blastopore, the 

 external is the ectoderm or outer germ-layer, the internal the entoderm 



or inner germ-layer. In the gastrula we meet for 

 the first time the formation of germ-layers, i.e., 

 the formation of definite embryonic layers marked 

 off from each other, from which organs arise by 

 differentiation. 



Invagination. The gastrula is formed from 

 the blastula by imagination (fig. 107, A). The 

 result is the same as when one side of a hollow 

 rubber ball is pressed into the other; the region of 

 vegetative cells gradually sinks in and becomes sur- 

 rounded by the cells of the animal pole (fig. 107, B). 

 Thus there arises, in addition to the cleavage 

 cavity, a new cavity, the anlage of the lumen of the 

 digestive tract; this increases and finally obliterates 

 the cleavage cavity, so that the invaginated part 

 of the blastoderm, the entoderm, becomes pressed 

 against that which remains external, the ectoderm. 



FIG 107 Gastrula- 

 tion of Amphioxus 

 (after Hatschek). The 

 animal pole above, the 

 vegetative below, in 

 comparison with fig. 

 1 01. In .4 the cells of 

 the vegetative pole are 

 beginning to sink in; 

 B, the invagination 

 completed, the cleav- 

 age cavity reduced to 

 a slit between the ento- 

 derm (en) and the ecto- 

 derm (e&) ; o, blastopore. 



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 say that the gastrula stage has been discovered in 

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

 that the yolk-material finds lodgement principally in 

 the entodermal cells. In most ccelenterata the gastrula 



is formed in another and apparently more primitive way. Isolated cells from 



the wall of the blastula migrate into the segmentation cavity and form a mass. 



In this a cavity is hollowed out and then a mouth breaks through, the result 



being structurally a gastrula. 



Epiblast and Hypoblast. For outer and inner germ-layer the terms epi- 



blast and hypoblast, upper and lower germ-layer, have been much used. Other 



terms for the two germ-layers are entoblast andectoblast. 



Formation of Mesoderm. The Mesenchyme. Many lower animals 

 e.g., most ccelenterates, have in general only two germ-layers. When 

 these are laid down there begins immediately the differentiation of muscle 

 and nerve fibres and the other histological changes of the cells, as well as 

 changes of form, by which the gastrula becomes the adult. In higher 

 organisms, on the other hand, before further differentiation begins, there 



