NO. 2 ENCYCLOPEDIA OF INSECT ANATOMY — SNODGRASS IJ 



sac and a posterior narrow part or rectum proper. The terminal 

 opening is the functional anus. 



Gastrula: The Greek word gaster really means the paunch or 

 belly, but as used in anatomy the gaster is the stomach. Gastrulation, 

 therefore, should mean stomach-formation regardless of the method 

 of formation, 



Borradaile and Potts ( Invertebrata, p. 127) state: "Every triplo- 

 blastic animal passes through a stage — the gastrula — in which it con- 

 sists only of ectoderm and endoderm. Save in this essential feature, 

 the gastrulae of different animals may be extraordinarily unlike, and, 

 especially when the animal is developed from a very yolky ^gg, they 

 are sometimes very difficult to recognize as such; but where the 

 gastrula is well formed, as in the familiar development of Amphioxus 

 or in that of a starfish, its two-layered wall may always be found to 

 contain a cavity, the archenteron, which possesses a single opening, 

 the blastopore." 



The first development of a metazoic egg commonly leads to a hol- 

 low mass of cells known as the hlastiila. If the blastula represents a 

 free-living ancestral form it probably obtained its food from the 

 water through its surface cells. If it commonly lived on the bottom 

 it would be natural that the cells of the underside would become 

 specialized for ingestion and digestion of food material. Then it 

 would be a further advantage if this surface should sink into the 

 blastula. Thus the animal would become a two-layered sac, the 

 cavity of which would be the primitive stomach or archenteron, the 

 opening of which is the blastopore. The outer cell layer becomes the 

 ectoderm, the lining of the stomach becomes the endoderm. 



In a few of the metazoic animals the stomach is formed during 

 embryonic development by introversion of the ventral wall of the 

 blastula. In an elongate animal the blastopore becomes divided into 

 a mouth and an anus. This stage in the ancestors of the arthropods 

 has been thought to be well represented in the onychophoran embryo 

 which presents a median ventral groove that closes between the two 

 ends. However, Manton (1949) finds that this groove is not the 

 elongated blastopore because its formation does not give rise to the 

 endoderm; the endoderm is proliferated internally from a generative 

 area behind the groove. It is this generative area which thus repre- 

 sents the true blastopore. The mouth-anus groove, therefore, is a 

 secondary formation, but evidently it must be formed in some way 

 from the blastopore. 



The method of endoderm formation by introversion is commonly 



