OF ARTS AND SCIENCES. 35 



of the ovum. In one case there would be a simple division of ma- 

 terial ; in the other, a histogenetic sundering of substances, which 

 ordinarily comes as one of the concluding events of cleavage. The 

 natural order is unquestionably two meridian cleavages followed by 

 an equatorial or horizontal cleavage ; and we have yet to see de- 

 cisive evidence that this order has been completely inverted in any 

 single instance. Polar segregation has been carried to such an ex- 

 treme in some ova that it practically amounts to a complete separa- 

 tion of food-yolk from the formative material ; and, if we mistake 

 not, the line of separation thus established has, in a few instances, 

 been mistaken for an equatorial cleavage. Something of this kind 

 has happened in the case of Nassa and Crenella, and we venture to 

 suggest that it may have occurred in that of Balanus and Scalpel- 

 lura.^^ 



In a single octavo plate Haeckel ^* has described the development 

 of a gastropod (^T?-ochus ?) and a cha^topod {Fabricia) as far as the 

 gastrula stage. He speaks of an axis which has its poles in the 

 " animal " and " vegetative " halves of the ovum, and which becomes 

 characterized either before or during the process of cleavage. The 

 cleavage of the gastropod ovum conforms to the rule befoi'e stated ; 

 while that of the chtetopod, to all appearances, proceeds on a funda- 

 mentally different plan. In Fabricia the first cleavage-plane appears 

 to be an equatorial plane, separating the ectodermic from the ento- 

 dermic pole. As Haeckel's figures are designed to illustrate the 

 formation of the gastrula rather than to give the details of cleavage, 

 it would be unjust to criticiee them too closely in the latter respect. 

 Before, however, Fabricia is put down as a real exception to the 

 general rule, one or two points must be cleared up by further investi- 

 gation. It is by no means certain that the first cleavage-plane is not 

 meridian, instead of equatorial as it appears to be. The relation of 

 the polar globules to the plane of division would decide the question. 



Haeckel's views are, however, irreconcilable with those here main- 

 tained ; for he states that the first cleavage-plane generally divides the 

 ovum into two uidike parts, " a smaller animal cell, the mother-cell of 

 the cxoderm, and a larger vegetative cell, the mother-cell of the ento- 

 derm"; and cites Fabricia, the rotifera, and the gephyrea as examples. 



3^ Rabl. "Entwick. d. Tellersclinecke." Morpliol. Jalirb., V., pp. 573-584, 

 1870. 



33 Lang. " Die Dotterfurchung von Balanus." Jena Zeitschr. f. Naturw., 

 XII., p. 671, 1878. 



31 Ilueckel. No. 3, p. 425, PI. XXIV. 



