60 PROCEEDINGS OF THE AMERICAN ACADEMY 



July 27, ova and spermatozoa of the Blackfish were mixed. The 

 ova proved to be immature, the cleavage halting at the 8-cell stage. 

 The interesting feature of the first cleavage was, that the two blasto- 

 meres at the close of the division assumed a perfectly circular outline, 

 and separated so far that a distinct space was left between them. 

 During the second cleavage, the two blastomeres came in contact, but 

 did not flatten against each other, as they ordinarily do. The third 

 cleavage was very feeble, and was not fully completed. 



During the sixth division, which results in the 64-cell stage, we 

 have noticed an irregularity of some importance. This irregularity 

 occurred in the division of the central cells. We found that in some 

 cases one or more floor cells divided horizontally, the upper cell taking 

 a position among the superficial cells ; in other cases, the reverse took 

 place, some of the upper cells dividing horizontally, and giving cells 

 to the floor layer. From such facts we may infer that there is no 

 histological difference at this time between the upper and lower layer. 



All these irregularities might be interpreted to favor the ideas 

 recently advanced by Pflliger ; but we cannot admit that they confirm 

 some of his more extreme conclusions. 



Other Cleavage Phenomena. — The cleavage-fulds ("' corona plica- 

 rum " of Max Schultze) are generally seen to best advantage during 

 the first and second cleavage ; but occur during the third, fourth, and 

 even the fifth cleavage as well. These folds are of the same nature 

 and appearance as those seen in the early stages of the frog's ovum. 

 They are the " Faltenkranz " of Reichert. They may be regarded as 

 an outward expression of the radial phenomena which accompany 

 cleavage, as has been suggested by one *^ of us, and as maintained by 

 Van Bambeke.^" 



Three classes of vacuoles are seen in our mounted preparations. 

 The first are thin lenticular spaces, bounded by the cleavage-faces of 

 contiguous blastomeres. These are few in luimber, and reappear 

 in successive stages, particularly the earlier ones. These vacuole- 

 like spaces are present in the living ovum. They are filled with a 

 fluid which does not stain. The second class of vacuoles are small, 

 round or semicircular, and arranged along either side of the ex- 

 ternal cleavage-lines. They are not found on the inner surface, nor 

 at points intermediate between this and the outer surface. Their 



-•s C. O. Whitman. "Embryology of Clepsine." Quart. Journ. Mic. Sc, 

 1878, p. 41. 



^ Ch. van Bambeke. " Nouvellcs Recherches sur I'Embryologie des Batra- 

 ciens." Arch, de Biol., I., p. oGO, 1880. 



