156 



GENERAL PRINCIPLES OF ZOOLOGY. 



4. Formation of the Germ-layers. 



Gastrula. Besides the blastula there is still a second stage of 

 development, the gastrula or the two-layered embryo, which is 

 common to all the Metazoa. This stage is 

 understood easiest in the case of eggs which 

 have an equal cleavage (fig. 105, Z?); here it 

 has the form of a double-walled cup with a 

 wider or narrower mouth. The cavity of the 

 cup (the primitive digestive tract or arclien- 

 teroii) is the beginning of the most important 

 part of the digestive system; the opening is 

 the primitive mouth or blastopore (iwostoma). 

 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 forma- 

 tion of definite embryonic layers marked off 

 from each other, the cells not yet differen- 

 tiated, from which organs arise through 

 organological and histological differentiation. 

 Invagination. The gastrula is formed 

 from the blastula by invagination (fig. 105, 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 vegetative 

 cells gradually sinks in and becomes surrounded by the cells of the 

 animal pole (fig. 105, B}. Thus there arises in the egg, in addi- 

 tion 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 the part which remains 

 external, the ectoderm. 



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 entodermal cells. 



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

 epiblast and hypoblast, upper and lower germ-layer, have been much 



FIG. 105. Gastrulation 

 of AmpJiioxus. (After 

 Hatscnek )Theanimal 

 pole here is above, and 

 the vegetative pole 

 below, in comparison 

 with fig. 93. In fig. 

 A the cells of the 

 vegetative pole are be- 

 ginning to sink in; B, 

 the invagination com- 

 pleted, the cleavage 

 cavity reduced to a 

 slit between the ento- 

 derm (en) and the ecto- 

 derm (ek) ; o, blasto- 

 pore. 



