GASTRULATION, FORMATION OF GERM LAYERS 79 



cells which contribute to the formation of the mid-gut epithelium, and 

 Hirschler (1905, 1909o, 19096) makes a similar claim for some Lepidop- 

 tera and Coleoptera (Catocala, Donacia, Gastroidea). Hammerschmidt 

 (1910) asserts that in Carausius (Dixippns) only the middle strand is 

 concerned in mid-gut formation, the bipolar rudiments being absent, a 

 statement in part confirmed by Leuzinger and Wiesmann (1926). Pater- 

 son (1935) found a similar condition in the beetles Corynodes pusis and 

 Euryope terminalis. A comparative study of these and some other 

 forms indicates that all stages exist in the development of the nonmeso- 

 dermic component of the inner layer: the bipolar rudiments alone, the 

 bipolar rudiments connected by a middle strand, the middle strand 

 alone, or any or all of these parts breaking down in the yolk. Perhaps a 

 majority of present-day embryologists support this view regarding the 

 development of pterygote insects. 



In the foregoing tabulation no mention is made of the old suggestion 

 that the mesoderm may play a part in the formation of the definitive 

 mid-gut epithelium of insects, since the idea is doubtless no longer 

 entertained by embryologists. 



Recently there has been an inclination on the part of some workers 

 to follow the lead of experimental embryologists as expressed by Richards 

 (1932) in the words: 



So the experimental embryologists, notably the workers on Amphibia, dis- 

 tinguish between "prospective significance" and "prospective potency" and 

 bring the idea in of "time of determination," etc. Pending pertinent experi- 

 mental data, we are forced to one of two conclusions: either the mid-gut epithe- 

 lium, despite its final similarity within individual orders of insects, may arise in 

 some cases as an entodermal derivative and in other cases as an ectodermal 

 derivative; or else we must look upon the determination of the mid-gut as a 

 purely physiological process — to borrow Driesch's terminology, "as a function of 

 the position within the whole." That is, that whatever cells in each particular 

 case happen to be appropriately located at the time when the entodermal strands 

 are due to be formed, those cells will be "determined" to form the mid-gut 

 anlage and subsequently differentiate into the structure typical of mid-gut epi- 

 thelium. The question is purely a matter of the time of the determination of the 

 parts involved. Experimental embryologists are inclined to accept the latter 

 interpretation as superior to the former in the light of the great diversity shown 

 by closely related insects and from the standpoint of modern views of develop- 

 mental processes in general. 



In view of the still prevailing diverse opinions regarding the develop- 

 ment of the germ layers as outlined above it is apparent that it would be 

 futile at present to attempt to formulate a theory that will satisfactorily 

 explain all established facts. The endeavors that have been made to 

 reconcile the several existing theories have not proved wholly convincing. 



