OF ARTS AND SCIENCES. 57 



neither arise de novo, nor from the division of the first cleavage- 

 ampliiaster, but in cells belonging to the margin of the blastodisc or 

 blastoderm, which have at first well circumscribed boundaries. Some 

 time before the embryonic ring appears, the inferior marginal cells 

 flatten and form a wreath around the blastodisc. From this time on- 

 w^ard these cells are entirely distinct from the blastoderm. Of their 

 further history and significance something still remains to be said in 

 the sequel. 



It is hardly necessary here to call attention to the general impor- 

 tance of the discovery of the precise origin of this peculiar cell-layer, 

 the history and meaning of which have so long been a standing puzzle, 

 forming one of the greatest obstacles in the way of understanding the 

 germ-layers of the vertebrates. We hope to be able to show later that 

 this history is equally applicable to other meroblastic vertebrate ova. 



T7ie Order and Direction of Cleavage-planes. — The first four cleav- 

 age-acts, ending with the 16-cell stage, present some points of interest 

 in addition to those already discussed under the head of " general re- 

 marks on the ovum." The direction of the first cleavage-plane is that 

 of the axis of the ovum, whether this be vertical or not. When the 

 ovum assumes its normal position of equilibrium, the axis is vertical, 

 or nearly so, in the majority of cases ; but this position is not uni- 

 versal, even among the teleostei. Gravitation of course influences 

 the ovum as a whole, and may thus be said to control, indirectly, the 

 directions of the cleavage-planes ; but the idea that the first or auy 

 subsequent plane is vertical simply because this is the direction of 

 gravitation, is in plain contradiction with the fact that cleavage-planes 

 may form all possible angles with the vertical plane. 



The succession of cleavage-planes, at least the earlier ones, in sjiite 

 of all irregularities, presents a general uniformity or order. Is this 

 order predetermined, von vorn herein ? and how far is it allowable to 

 speak of homologous cleavage-planes ? The correspondence between 

 the first cleavage-plane and the median plane of the embryo, which 

 has already been ascertained in a considerable number of cases, favors 

 the opinion that this cleavage-plane is homologous in at least all ani- 

 mals with bilateral symmetry. The evidence is, however, very far 

 from being complete. It does not follow, because one cleavage-plaue is 

 homologous, that all the rest must be so. In comparing the teleostean 

 and amphibian types of cleavage, we find no ditficulty with the first 

 two meridian planes of cleavage ; but the homology of the third and 

 of the fourth is not so obvious, while beyond this one would hardly 

 venture to compare individual cleavage-planes. Rauber is the only 



