No. I.] CONTRIBUriOX TO nXSECT EMBRYOLOGY. 51 



gastrula, which is so very short and broad, we may suppose 

 that the oral and anal entoderm-centres are really continuous, 

 covering the floor of the blastopore from end to end. In sec- 

 tions, it is true, I failed to detect any differentiation of the 

 cells forming the walls of the furrow, into entodermal and 

 mesodermal elements, but this would also be the case in the 

 elongated gastrulas of other insects in a correspondingly early 

 stage {XipJiidiiini, DorypJiord). As favoring a purely mechan- 

 ical separation of an originally single entoderm Anlage, it may 

 be noted that the most rapid elongation of the germ-band 

 occurs at a time when the entoderm is differentiating from the 

 mesentoderm Anlage. There is probably more than an acci- 

 dental correlation of these two processes. During this time 

 some germ-bands {Doryphora, XipJiidiimi) double their length. ^ 

 Inasmuch as the lengthening of the superficial layers of the 

 embryo is much more rapid than the differentiation of the 

 entoderm, this germ-layer must lag behind. In most insects 

 the embryo is at the time of its greatest elongation much longer 

 than the yolk-mass and must again shorten to the length of this 

 mass, so that a rapid proliferation of the entoderm may be 

 superfluous, since this layer would have to readjust itself to 

 the yolk with the contraction of the embryo. It may, therefore, 

 be an advantage for the entoderm to be somewhat retarded in 

 its growth. In the Orthoptera, where the embryo lengthens 

 rapidly, shortens, and then lengthens again to envelope the yolk 

 we may suppose, for reasons to be given in the sequel, that 

 yolk has been acquired. This seems also to be suggested by 

 the histological structure of the embryonic entoderm; this 

 layer consisting of large polygonal cells in the Coleoptera, 

 which have only a medium amount of yolk, while in the 

 Orthoptera the attenuate entoderm-bands consist of a very 

 few flat cells.2 



It is probable that when more forms have been carefully 



1 Blatta forms a very rare exception in this respect. 



2 A very similar condition may be observed in the case of the blastoderm. In 

 the Coleoptera and Diptera, which have a medium or small amount of yolk, the 

 newly formed blastoderm is a deeply columnar epithelium ; in the Orthoptera it is 

 a true pavement epithelium. 



