Hyatt.] 56 [March 5, 



formed exceedingly clear. Here again, however, the fact of a 

 sequence in the phenomena is unsustained by the morphological 

 evidence of direct union, and stress is laid upon the mere suc- 

 cession, which as in the case of Ostrea might be considered as a 

 reversion, or as an acquired peculiarity of this form. 



These examples are also important in another sense, since 

 they, as now understood, are directly opposed to the theory 

 advocated, farther on, that the agamic occurrence of polar glob- 

 ules in Metazoa may be explained by the law of concentration 

 and acceleration in development. 



The absence of distinct polar vesicles in several isolated instan- 

 ces as well as generally in the Arthropoda and Rotifera is noted 

 by Balfour, Mark and others. Prof. Mark (Bull. Mus. Comp. Zool. 

 vol. vi, p. 548), is disposed to favor the views of the universality 

 of the formation of polar globules as homologous processes. 

 " There seems to be little doubt that the elimination of portions of 

 the substance of the germinative vesicle as described by Balfour 

 for Elasmobranchs, by Oellacher for bony fishes and birds, and 

 by Van Bembeke and Hertwig for Amphibia represents in some 

 hitherto unexplained manner the formation of polar globules." 

 According to Ryder (Embryog. Osseous Fishes), a slight promi- 

 nence arises on the germinative membrane opposite the micropyle 

 in the egg of the Cod, which, though uncertain, is possibly the 

 homologue of the polar vesicle. The polar globules have not 

 been found in the Sponges though no direct search has as yet 

 been made which would justify one in assuming that they may 

 be absent in this group, and we know very little about the ear- 

 liest stages of the ova, during and immediately preceding impreg- 

 nation. 



Parthenogenetic development of eggs taking place after the 

 expulsion of the polar globules does not seem to be explicable 

 upon a gonoblastic basis, and would appear to tell against any 

 theory of the essential nature of the processes of fecundation. 

 If true, it indicates the primitively agamic nature of all phe- 

 nomena of growth by segmentation and thus reduces the act 

 of impregnation to a secondary or acquired habit arising from 

 the differentiation of the sexes. 



Balfour quotes the researches of Butschli, Strasburger and 

 Hertwig in support of his view of the polar globules, and then 



