52 WHEELER. [Vol. VIII. 



studied a method of entoderm formation midway between the 

 unipolar and bipolar methods will be found to obtain in some 

 insects. We must admit that a contribution of elements* to the 

 entoderm from the interpolar region of the furrow is not with 

 certainty precluded in several of the species which have been 

 studied. Thus Heider, ('89) while inclined to believe that such 

 a contribution does not take place in the anterior portion of 

 the germ-band, believes that it may take place in the posterior 

 abdominal segments. Cholodkowsky ('91®) is inclined to accept 

 a still more diffuse origin for the entoderm. " Untersuchungen 

 zahlreicher Schnittserien machen es wahrscheinlich, dass an 

 verschiedenen Stellen des Keimstreifens sich einzelne Zellen 

 vom ausseren Blatte abspalten und an der Bildung des inneren 

 Blattes betheiligen, so dass die Entstehungsart des letzteren 

 sehr complicirt erscheint." 



Starting with the same postulate as Kowalevsky, viz: that 

 the bipolar is derivable from a unipolar condition of the ento- 

 derm — Cholodkowsky ('91®' '91^) proceeds to account for this 

 phenomenon in a very different way from his compatriot. He 

 takes the small, round blastopore of Astacns, stretches it till it 

 equals the insect blastopore in length, introduces a number of 

 modifications — such as the median groove and the pairs of 

 lateral depressions and believes that he has found an explana- 

 tion " sehr klar und ungezwungen " for all the different blasto- 

 pores, not only in the Insecta, but also in the meroblastic eggs 

 of vertebrates. It was Kleinenberg who said: "Gewagte Hypo- 

 thesen, kiihne Schlusse niitzen der Wissenschaft fast immer, 

 die Schemata schaden ihr, wenn sie die vorhandenen Kenntnisse 

 in eine leere und dazu noch schiefe Form bringen und bean- 

 spruchen tiefere Einsicht zu geben." The latter part of this 

 aphorism seems to be particularly applicable to Cholodkowsky's 

 exposition. As Graber ('9l) has briefly pointed out, there are no 

 grounds for comparing the Astacns blastopore with the entire 

 insect blastopore. In the Decapods this orifice is confined to 

 the anal region and if comparable at all to the median furrow 

 in insects, must be compared with the caudal entoderm pole. 

 This is all that is admissible, since the mesoderm of the 

 Decapoda arises from the anterior lip of the blastopore and 



