IV. Embryology. 



47 



wanting; its place is occupied to a certain, extent by food-yolk. 

 Invagination takes place as in the typical gastrula (1, p. 40) and the 

 food-yolk is gradually absorbed by the cells. 



Fig. 36. Development 

 of the ovum of a Crus- 

 tacean (sections). Bl 

 Tolk, eh epiblast, en 

 hypoblast, m mesoblast, 

 blastopore. — After 

 Haeokel. 



From the above it follows that a gastrula, always formed in 

 essentially the same way, is found almost universally (when the 

 examination has been sufficiently thorough), as a result of the 

 earliest development of the egg. 



An aberrant gastrula-formation oociirs in many Hydi-ozoa. No invagination 

 takes place here, but at different points in the blastula, cells break away from 

 the rest and wander into the segmen- 

 tation cavity to constitute the hypoblast A B 

 (Fig. 37 A). Sometimes the prolifera- 

 tion of cells is limited to one side of 

 the blastula (B). Possibly the mode 

 of hypoblast formation figured in A is 

 primitive: from this B can easily be 

 derived, and it may be a transition to the 

 typical gastrula, which also occurs in the 

 Hydi'ozoa. 



Between the two primary germ- 

 layers, of which the gastrula 

 consists, a third layer, the meso- 

 blast or middle germ-layer, is formed in most animals (Ooelentera 

 excepted), very soon after, or even during, the formation of the gastrula. 

 There is no general type of formation recognisable for this, as there is 

 for the gastrula. In some cases it is formed, for example, by some 

 few cells lying at the blastopore, at the boundary between the epiblast 

 and the hypoblast, breaking from their connection with the rest, 

 pushing between the two sheets, and after repeated division, ex- 

 tending between them as an independent layer (Figs. 30, 31). In 

 other cases it originates as a double fold or sac of hypoblast, which 



Fig. 37. Diagrammatic sections ex- 

 plaining the formation of the gastrnla in 

 certain Hydrozoa. 



