No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 53 



proliferates headward. That such is its origin has been shown 

 by Bobretzky and Reichenbach for Astaciis ('86), by Paul 

 Mayer for Eiipagtirns (^n), and by Bumpus for Hoinanis ('9l). 

 To Cholodkowsky both the extent and position of the blastopore 

 are of little consequence as is abundantly evident from his 

 reply to Graber's well-founded objection. It is this very neglect 

 of what are generally and, I believe, rightly considered two of 

 the most important matters in the discussion of the germ- 

 layers, which stamps Cholodkowsky's hypothesis as superficial 

 and inadequate. 



There is, however, one redeeming suggestion in his hypo- 

 thesis, viz: that the diverging grooves at the posterior end of 

 the blastopore in insects may correspond to the " Sichelrinne " 

 of vertebrates. Certainly the relations of the grooves to the 

 median furrow in Xiphidimn (see Fig. i.) closely resemble in 

 surface view the relations of the "Sichelrinne" to the primitive 

 streak in the chick as figured by Koller ('81) and in the triton 

 as figured by Oscar Hertwig ('90, p. 99). 



While most investigators probably agree with Kowalevsky 

 and Cholodkowsky in deriving the bipolar from a unipolar con- 

 dition of the entoderm. Patten does not share this view ('90). 

 In his opinion, which is based on Kleinenberg's interpretation 

 of the gastrula, the blastopore is restricted to the oral region, 

 and such depressions as occur at the posterior end of the germ- 

 band, as well as the formation of teloblasts in that region, are 

 supposed by him to have no connection with the blastopore, 

 but to be merely the instruments of unipolar growth. "The 

 Arthropod body represents an outgrowth from the trochosphere, 

 but the trochosphere itself, the coelenterate stage, has disap- 

 peared. Hence there is no such thing as a gastrula in 

 Arthropods and strictly speaking, no germ-layers." It is clear 

 that this view must stand or fall with Kleinenberg's theoretical 

 conclusions on which it is based, and we may venture to say 

 that E. B. Wilson's recent work ('9o) has rendered this founda- 

 tion very insecure, notwithstanding Patten's rather confidant 

 assertion that "in Lopadorhyuc/uis it is certain that the greater 

 part of the mesoderm arises from the ectoderm at the growing 

 tip of the tail, and has nothing to do with primitive mesoderm." 



