156 



GENERAL PRINCIPLES OF ZOOLOGY. 



4. Formnflon of the fierm-hiuers. 

 Gastrula. — Besides the LListula there is still a second stage of 

 developmeut, the gastrula or the two-layered omhryo, which is 

 common to all the Metazoa. This stage is 

 understood easiest in the case of eggs which 

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

 has the form of a doulile-walled cup with a 

 wider or narrower mouth. The cavity of the 

 cup (the primitive digestive tract or archen- 

 teroii) is the beginning of the most important 

 part of the digestive system ; the opening is 

 the primitive mouth or Uastopore (prostoma). 

 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 ofE 

 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 hj 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 aiid finally obliterates the 

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

 the entoderm, becomes ^n'essod against the part which remains 

 external, the ectoderm. 



Modified Modes of Gastrulation. — In the case of eggs with much food- 

 yolk tlie relation of the structure and of tlie mode of formation of the 

 gastrula is more difficult to understand. Hero, however, it is sufficient to 

 mention tlio fact tliat tlie gastrula stage has fortunately been discovered 

 for almost all eggs witli a great quantity of food-yolk, and that the yolk- 

 material finds lodgment principally iu tlio entodermal cells. 



Epiblast and Hypoblast. — For outer and inner germ-layor the terms 

 epibiast and liypobhisl, upper and lower germ-layer, have been much 



Fig. 1115. — (.Tastrulation 

 of ,lijm/iM).rM.s. (After 

 Ha tscnek.) The animal 

 pole here is above, and 

 the ■N'e.s^etative pole 

 Ijelow, in conipai'ison 

 with fig. 93. In fig. 

 A the cellH of the 

 vegetative pole are be- 

 .giiining to sink in; B, 

 the invagination com- 

 pleted, the cleavage 

 cavity reduced to a 

 slit betvpeen the ento- 

 derm Icn) and the ecto- 

 derm ick) ; o, blasto- 

 pore. 



