32 PROCEEDINGS OF THE AMERICAN ACADEMY 



The teleostean ovum afForils a beautiful illustration of what Lankes- 

 ter -^ has designated as " precocious segregation " ; for here tlie sepa- 

 ratiou of germinal material from food-yolk becomes complete, or 

 nearly so, before the cleavage process begins ; and even the blastodisc 

 and the periblast, at first continuous, soon part company without the 

 direct intervention of cleavage. In these respects this ovum repre- 

 sents a higher type than does the elasmobranch ovum ; but there is 

 another respect in which the reverse seems to be true. The fact that 

 the entire germ-ring enters directly into the embryo certainly points 

 to a more primitive mode of embryonic formation than is seen in the 

 elasmobranch ovum. In this particular the elasmobranch ovum repre- 

 sents an intermediate condition between the teleostean and the avian 

 ovum. How is this fact to be reconciled with the opinion that the 

 teleostean ovum represents a later type than that of the elasmobranch ? 

 We are of the opinion that it can be best explained on the hypothesis 

 first suggested by Bairour,(-°) that the teleosts are derived from a type 

 offish witli a much larger ovum. The presence of an enormous mass 

 of food-yolk in the elasmobranch ovum retards the closure of the 

 blastopore, and the general effect is the same as if the formation of 

 the embryo had been greatly accelerated. The result is that the 

 embryo is already formed and constricted off from the yolk before 

 the blastopore closes. In precisely the same way we may explain the 

 still more extreme case of the chick. Assuming that there has been 

 a large reduction of food-yolk in the ovum of the osseous fish, we 

 may safel}'^ conclude that the closure of the blastopore has been corre- 

 spondingly hastened ; and it is the comparatively early closure which 

 renders possible the inclusion of the entire germ-ring in the embryo. 

 Thus the apparently earlier type of development exhibited in osseous 

 fishes may be explained as a case of reversion. 



A very marked polar differentiation characterizes the mature 

 teleostean ovum. Ilatschek'-* was the first to call attention to the 

 universality and the early ajipearance of polarity in the egg; and 

 Balfour^® has pointed out the influence of the polar concentration of 

 the vitellus in determining the various forms of cleavage. Von Baer^ 

 defines very clearly the axis of the frog's ovum, and its relation to the 



^ Lankestcr. Notes on Embryology and Classification, for the Use of Stu- 

 dents. London, 1877. 



25 Hatschek. " Pedicellina." Zeitschr. f. Wiss. Zool., XXIX.. p. 502, 1877. 



26 Balfour. Comparison, etc. Quart. Journ. Micr. Sci., p. 210, 1875. 



27 K. E. von Bacr- "Die Metamorphose des Eies der Batrachier." Miiller's 

 Arch., p. 484, 1834. 



