2^8 KNOWER. [Vol. XVI. 



the outcome of concentration of the cells of the disc toward 

 the center. The relation of such a manner of formation of the 

 under-layer to that generally described for insects is interesting 

 to consider. This process does not appear to me to be derived 

 from an invagination as a slurred gastrula. It is rather a 

 method of delamination, where th«re is a further tendency in 

 the lower-layer cells to collect toward a middle point. 



A similar method has been described for Crustacea, Arach- 

 nids, and Myriopods, and all of these facts, taken together, lend 

 weight to Heymons's contention that an indefinite migration 

 below is the more primitive method of forming the under-layer 

 in insects. 



Heymons's explanation of the gastrula groove commonly found 

 in insects, however, requires examination. 



He does not attribute to such invaginations the significance 

 of a process of gastrulation. From his standpoint the invagi- 

 nate groove (which, as Wheeler points out, is so universally 

 present among insects, and so essentially involved in the estab- 

 lishment of the under-layer) is a mechanical process and inde- 

 pendent of the formation of mesoderm or endoderm. 



I do not see the strength of this position. 



In so far as this author finds the diffuse method of the origin 

 of the mesoderm in certain Orthoptera the primary one, and 

 offering a favorable basis for the origin of an invaginate gas- 

 trula, he seems justified. 



I cannot, however, take the further step with him and dis- 

 miss the invaginate gastrula, found so universally among insects, 

 as no gastrulation, but as simply a result of the crowding of an 

 aggregation of cells at one point. Though we still have such 

 an aggregation in the Termite, it has not in this group led to 

 invagination as a mechanical necessity. In the place of invagi- 

 nation there is simply a crowding of certain ectoderm cells, 

 arising at irregular points, below into a solid plug extending 

 down into the yolk. 



As far as our understanding of mechanical forces and their 

 necessary results goes, the reason is not clear, without further 

 addition, why the mesoderm came in other forms to arise in a 

 groove instead of continuing to wander below in a solid mass. 



