598 WILSON. [Vol. V 1 1 1 . 



(perhaps during the division of the cells in this region) for a 

 definite pair of large cells.^ 



But apart from the origin of the mesoblast the differentiation 

 of the germ-layers in Amphioxiis differs widely in other respects 

 from that of annelids. In the latter forms the cleavage is 

 always unequal (even in Polygordiiis, as shown by my own un- 

 published observations), and after the formation of the first 

 four cells several groups of micromeres, each consisting of 

 four, are formed from them before the gastrulation takes place 

 (there are three of these groups in Nereis, four in Psygmo- 

 brancJms and Ltinibriconereis, five in Aricia and Polymnia ; see 

 No. 35, appendix). In all cases the first three groups give 

 rise to ectoblast ; of the fourth group one blastomere gives 

 rise to mesoblast and three to entoblast ; while the fifth group 

 are pure entoblast. 



Amphioxus diverges from the annelid at the fourth cleavage ; 

 for the smaller cells (the "secondary macromeres" A2, B2, 

 C2, D2) separated at that time from the primary macromeres 

 are not ectoblastic but mixed in character, giving rise both to 

 entoblast and ectoblast and perhaps also to mesoblast. These 

 cells, as I have shown, remain throughout the development in 

 contiguity with the primary macromeres, forming with them 

 the ento-mesoblast plate, and like them continue to bud forth 

 micromeres until a late stage. Thus, in annelids, the fourth 

 cleavage is qualitative (using the word in a purely prospective 

 sense) while in Amphioxtis, although unequal as in the annelid, 

 it is quantitative only. Whether this difference may hereafter 

 be found to be of any phyletic importance is an open question. 



B. On Normal Cleavage-Types. 



I have elsewhere pointed out (35) that irrespective of yolk- 

 storage, or of the mode of gastrulation, the early stages 

 of cleavage in the animal ovum exhibit three more or less 



^ These conclusions were laid before the American Morphological Society at the 

 December meeting, 1892. The same result was published by Lwoff in a paper 

 received soon afterwards, and I know of at least two other admirable observers 

 who have sought in vain for the mythical pole-cells. They will no doubt long 

 continue to haunt our text-books, where they are already quite at home ; but it 

 is to be hoped that the ghost may in time be laid. 



