146 



GENERAL PRINCirLES OF ZOOLOGY 



narro\Yer mouth. The cavUy of the cup (the priniitivo digestive tract or 

 circhfiilcro)!) is the beginning of the most important part of the digesti\e 

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

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

 external is the ccloih-rm or outer germ-layer, the internal the entoderm 

 or inner germdayer. Li the gastrula we meet for 

 the lirst time the formation of germdayers, i.e., 

 the formation of defmite embryonic layers marked 

 off from each other, from ^^l■uch organs arise by 

 differentiation. 



Invagination. — The gastrula is formed from 

 the blastula by iiivaginatio)! (lig. 107, A). The 

 result is the same as when one siile of a hollow 

 rulilter ball is pressed into the other; the region of 

 vegetative cells gradually sinks in and Ijccomes sur- 

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

 Thus there arises, in addition to the cleavage 



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



Fig 107. — Gastrula- - ' - '',-„,,■ 



tion of A m pli io.xiis digestive tract; tJiis increases and tmally obliterates 



(after Haischck). The ^j^g cleavage cavity, SO tliat the invaginated part 



animal pole above, the ^ ^ ' 



of the blastoderm, the entoderm, becomes pressed 

 against that wltich remains external, the ectoderm. 



vegetative below, in 

 comparison with lig. 

 loi. In .t 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 en to- 



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 

 dillicult to understand. Here, however, it is sufticient 

 to say that the gastrula stage has been discovered in 



derm (en) and iheecto- almost all eggs with a great quantity of food-volk, and 



derm(.(i);<., blastopore, that the yoik-materialfinds lodgenient principally in 

 the entodermal cells. In most cadenterata the gastrula 



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



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



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



being structurally a gastrida. 



Epiblast and Hypoblast. — kor oiUer and inner germ-laver the terms e]ii- 



blast and ]i)-|iul)Uisl, uppci- and lower germ-la)-er, ]ia\'e been much used. Other 



terms for the two germ-layers are ciitobtast and cctobtast. 



Formation of Mesoderm. The Mesenchyme. — ISFany lower animals 

 e.g., most caienterates, have in general oi\ly two germ la^'ers. When 

 these are laid down there begins immediately the tlifferentiatioir of muscle 

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

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

 organisms, on the other hand, liefore further (.lil'fe -entiatiun begins, there 



