No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 45 



pleted and hence some time before the germ-layers are formed, 

 they cannot properly be assigned to the entoderm. ^ 



It is probably best to await the results of further investiga- 

 tion before deciding on the phylogenetic relations of the vitel- 

 lophags to the entoderm. Heider ('89) has also expressed 

 himself to this effect and I fully endorse his opinion when he 

 says : " Immerhin wird man vorlaufig iiber vage Vermuthungen 

 nach dieser Richtung nicht hinauskommen, und ist die Frage 

 nach der Auffassung der Dotterzellen bei dem Nachweise, dass 

 sie an dem Aufbau des Embryos keinen Antheil nehmen, 

 meiner Ansicht nach von geringerer Wichtigkeit." 



1 Besides these vitellophags which with Cholodkowsky ('91^) we may call the 

 primary yolk-cells — there are other cells which detach themselves from the blas- 

 toderm or embryo and enter the yolk. These Cholodkowsky calls secondary 

 yolk-cells. While the origin of the primary yolk-cells has been quite satisfactorily 

 demonstrated, this cannot be said of those of the second class. They appear to 

 descend into the yolk at different times in different species. Thus, according to 

 Patten {'84), all the cleavage products in Neophylax ascend to the surface, the 

 yolk-cells subsequently descending from the blastoderm. I claimed a similar total 

 migration of the cleavage products to the surface in Blatta ('89); Cholodkowsky, 

 however, claims that some of the cells never reach the surface, but remain in the 

 yolk. Be this as it may, in later stages I believe it can be shown that cells do 

 migrate into the yolk from the embryo and especially from the entoderm-centres. 

 This was shown by me to be the case in Doryphora, where many cells pass into the 

 yolk from either entoderm pole (PI. XIX, Fig. 82; PI. XX, Fig. 88). I have since 

 observed an exactly similar phenomenon in Telea polyphemus in a correspond- 

 ing stage of development. Graber, ('89, p. n) too, has made a similar observa- 

 tion on Melolontha, where he saw " vom invaginirten Blastodermwulst aus unter 

 lebhaften Theilungserscheinungen ganze Strome von Zellen in den Dotter hinein- 

 wandern, Zellen die freilich von den primaren, gleichzeitig vorkommenden und 

 auffallend grosskernigen Centroblastelementen ganz enorm verschieden sind, und 

 die sich iiberhaupt durch ihre ganze Beschaffenheit als unzweideutige Abkbmmlinge, 

 man konnte sagen Auswiirflinge eines wahren Keimblattes, erweisen." So far as the 

 migrant cells described in Doryphora are concerned, I am sure they come from 

 the entoderm. They occur only at or near the entodermal Anlagen and may be 

 traced from this germ-layer into the yolk. These cells are not actively dividing 

 like those described by Graber, but actively disintegrating. (May not Graber 

 have mistaken disintegration-figures for caryokinetic figures?) In somewhat 

 later stages no traces of these migrant cells are to be found. The yolk is 

 segmented at the time of their leaving the entoderm and their invasion appears 

 not to disturb in the least the activities of the vitellophags. Whether there 

 is any relation between these evanescent entoderm cells and the " secondary 

 mesoderm" of Reichenbach ('86), the "spores" of F. H. Herrick ('86), or the 

 " chromatin nebulae " of Bumpus ('91) is a question which cannot be answered at 

 present. 



