^6 MCMURRICH. [Vol. XI. 



pied a polar position. It will be seen that there is a large gap 

 in Bobretzsky's observations between his supposed earlier 

 stage and the next he describes, and I believe from my obser- 

 vations on an undetermined species of Onisacs that he is en- 

 tirely in error, that his disc represents a comparatively late 

 stage of segmentation, and that his first stage was an abnormal 

 Qgg. In other words, the Oniscus which I observed had a 

 typical centrolecithal segmentation closely similar to that of 

 Asellus, certain of the cells aggregating at an early stage to form 

 a blastoderm or disc, the rest of the yolk being covered by 

 scattered stellate cells whose existence Bobretzsky probably 

 overlooked. Nusbaum ('86), who also studied Oniscus nmrarms, 

 did not observe the segmentation stages, but states that, in 

 ova in which the disc was not yet present, the central portion 

 of the yolk stained deeply and was coarsely granular, and seems 

 inclined to regard this as indicating the presence at that point 

 of the "plasme formatif," though he was unable to distinguish 

 the first segmentation nucleus. 



Close on the heels of Nusbaum's paper appeared one by 

 Reinhard ("87) on Porcellio scaber, for which he described a 

 typical centrolecithal segmentation, the cells formed by the 

 division of the original centrally situated segmentation cell 

 gradually migrating towards the surface of the yolk, upon 

 which they are scattered, forming what Reinhard terms " In- 

 selchen." Up to this point this author's observations tally 

 perfectly w^ith my own, but his account of the formation of the 

 mes-endoderm is, I think, incorrect. He says : " Unter diesen 

 Inselchen konnen sich an verschiedenen Stellen mehrere Lagen 

 von Zellen befinden . . . Der Raum zwischen den Inselchen 

 fullt sich allmahlich an, und bald zeigt sich ein Theil der-Ei- 

 oberflache mit einer dichten Lage des Ectoblasts bedeckt. Die 

 unter dem Ectoblast liegenden noch indifferenten Zellen stellen 

 das primare Entoderm dar. Nur allmahlich differenziren sich 

 dieselben in Mesoderm- und Entoderm-Zellen." In how far 

 my results differ from these will be seen later. 



Roule has contributed several short notices on the develop- 

 ment of the Isopods, and especially of Porcellio. In the first 

 of these ('89) he describes the egg of Porcellio as having at 



