No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 49 



be unable to detect the outlines of the segments in the yolk of 

 eggs treated for half a day with Perenyi's fluid is not surpris- 

 ing, especially as this segmentation is of very short duration in 

 Blatta as in other Orthoptera. It is present, however, as I 

 have convinced myself from eggs mounted in toto and from 

 sections. 



If prolonged immersion in Perenyi's fluid can bring about a 

 complete fusion of the yolk-bodies and an obliteration of their 

 true structure, what must be its effect on the vitellophags 

 scattered through the yolk } And how much importance are 

 we to attach to Cholodkowsky's assertion that the fat-body, 

 blood-corpuscles and sexual-cells arise from the vitellophags, 

 and to the parablast theory as applied to the Blatta-ovwm. ? 



Let us return from this digression to the germ-layers. The 

 curious fact that the definitive entoderm of the Insecta arises 

 from two separate centres — one oral and the other anal — is 

 too recent to have given rise to much speculation. Since 

 the entoderm of other animals arises from a single centre it is 

 tacitly assumed that such must originally have been the case 

 with the Insecta, and that the present bipolar condition must be 

 due to secondary modification. Starting with this postulate, 

 there are, of course, many ways in which the bipartition of the 

 original unipolar entoderm may be supposed to have taken 

 place. Two of these possibilities are worked out in the hypo- 

 theses of Kowalevsky ('86) and Cholodkowsky ('9i^). 



Kowalevsky has expressed his views so clearly and con- 

 cisely that I cannot do better than quote his own words: 

 "Wenn wir jetzt versuchen, diese Bildung des Ento- und 

 Mesoderms bei den Musciden mit der Bildung dieser Blatter 

 bei anderen Thieren zu vergleichen, so sehen wir erstens, dass 

 hier auch eine Art sehr in die Lange ausgezogener Gastrula 

 entsteht, und dass aus dem eingestiilpten Teil das Ento- und 

 Mesoderm sich bildet. Also in diesen allgemeinen Zugen 

 finden wir eine Uebereinstimmung. Es scheint mir aber, dass 

 die Parallele noch weiter gezogen werden kann. Namentlich 

 wenn wir der Bildung des Ento-Mesoderms bei Sagitta uns 

 erinnern, so finden wir bei derselben dass der eingestiilpte 

 Teil des Blastoderms in drei parallele Sacke zerfallt, von 



