l8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



replaced by the internal proliferation of cells from the ventral sur- 

 face of the blastoderm. It is bewildering to read the various con- 

 flicting accounts of gastrulation in the insects as reported by differ- 

 ent writers and summarized by Johannsen and Butt (1941) and by 

 DuPorte (1960), but much of the confusion results from not recog- 

 nizing that introversion and proliferation may be just two super- 

 ficially different ways of forming the endoderm. 



Gastrulation: This is the process of formation of the stomach 

 irrespective of the method by which it is accomplished. The word 

 itself is derived from gastrula, the dimunitive form of the Greek 

 word gaster, which is used in anatomy for the functional stomach of 

 an animal. 



The first development of the tgg commonly leads to the formation 

 of a hollow mass of cells known as the hlastula. If the blastula rep- 

 resents a free-living ancestral form of the Metazoa, it probably 

 obtained its food from the water through its surface cells. If it com- 

 monly lived on the bottom of a body of water, the cells of the under- 

 surface may be supposed to have become specialized for the ingestion 

 and digestion of food material. It would then be an advantage if 

 these cells should sink into the blastula forming a cavity in which the 

 food could be carried about and more leisurely digested. This food 

 cavity is the primitive stomach or archentcron, the opening of which 

 is the blastopore. The outer cell layer of the body is the ectoderm, 

 the wall of the stomach is the endoderm. 



In the embryogeny of a few of the lower Metazoa the stomach 

 is formed by this method of introversion of the digestive cells of the 

 blastoderm. However, when the egg, as in most insects, contains a 

 large amount of yolk which becomes surrounded by the blastoderm, 

 gastrulation by introversion becomes entirely impractical. Aside from 

 the mechanical difficulties of invagination when the center of the 

 blastula is filled with yolk, introversion would place the yolk (food) 

 in the body cavity and outside the stomach. The insect embryo, there- 

 fore, cannot recapitulate the primitive method of stomach forma- 

 tion; it must adopt some other method. 



The ways by which the endoderm is formed by the insect embryo, 

 as reviewed by Johannsen and Butt (1941) and by DuPorte (1960), 

 are seemingly so various that it becomes bewildering to attempt to 

 interpret them all as derived from introversion or some modifica- 

 tion thereof. However, the process of infolding one wall of the 

 blastula may be replaced by the immigration of single cells. Such cells 

 are usually called yolk cells, but, since they presumably act as 



