MUSEUM OF COMPARATIVE ZOOLOGY. 427 



pressure of the embryo against the membrane directly above it ; but 

 we have not yet the proof that such is universally the case when polar 

 globules exist. The difference in specific gravity will presumably be too 

 little, in most cases, to cause any appreciable pressure in either direction 

 along the vertical axis. It is, however, quite another question whether 

 polar globules are coenogenetic phenomena. Rabl certainly deserves 

 credit for having turned the discussion concerning polar globules in a 

 phylogenetic direction, and, unsatisfactory as his protection theory 

 seems, it does not necessarily follow that there is no ground for his 

 claim that the polar globules are comparatively recent acquisitions. If 

 the globules were limited, as he claims, to eggs with unequal segmenta- 

 tion, there would certainly exist good reason to infer that they were 

 in some way adaptive acquisitions of this latter class of eggs. But the 

 following are a few of the many exceptions which make it improbable 

 that polar globules are coenogenetic adaptations to unequal cleavage -. 

 Hydra (Kleinenberg, 72, pp. 46, 47, 51) ; Lucernaria (Korotneff", 76, 

 p. 393) ; Hippopodius, Sagitta, and Echinoderms (0. Hertwig, 78"). 

 Indeed, if Strasburger is right in maintaining that the canal cells of 

 ; conifers, and equivalent cells of both lower and higher plants, are ho- 

 I mologous with polar globules, we must apparently go back to a very 

 I early point in the history of organisms to discover the origin and true 

 , significance of these cells, unless it is assumed they have been separately 

 acquired by the two recognized groups of higher organisms, 



Rabl has realized the sentiment of Von Baer's with which he closes 

 his last paper : " Irrige, aber bestimmt ausgesprochene allgemeine Re- 

 sultate haben durch die Berichtigung, die sie veranlassen, und die 

 scharfere Beachtung aller Verhaltnisse, zu der sie nothigen, der Wissen- 

 schaft fast immer mehr geniitzt, als vorsichtiges Zuruckhalten in dieser 

 Sphare." 



Stossich (76) entertains views of the morphology of the egg whicli 

 are at variance with well-established information. He is apparently 

 influenced in his opinion by the study of the germinative vesicle under- 

 going metamorphosis. 



The egg, he says, is a cell, but the nucleus of this cell is not the' 

 germinative vesicle ; it is the germinative dot, and within it may be 

 found the nucleolus. The body of the cell, i. e. the yolk, is composed 

 of two layers, — an external, adapted to the formation of granules, and 

 an internal (germinative vesicle), homogeneous and hyaline, in which 

 are contained the nucleus and nucleolus. The so-called germinative 

 vesicle is not, in his opinion, a vesicle having a proper membrane, 



