210 Mr. J. T. Cunningham on some 



he yolk ; but surely that fact is sufficiently obvious — what 

 else is the yolk for ? 



M'lntosh and Prince have not fully grasped the meaning 

 of my remarks concerning the segmentation in my paper "On 

 the Relations of the Yolk to the Gastrula." I said that the 

 first cleavage of the blastodisc into two cells represented 

 theoretically the division of the whole ovum into two similar 

 cells, each containing a cap of protoplasm and a large quantity 

 of yolk, although actually the two cells are continuous inte- 

 riorly and the yolk is continuous throughout. They say it is 

 difficult to maintain such a relation of blastomeres and yolk 

 when the morula is reached. By morula they apparently 

 mean the segmented blastoderm. But it is obvious enough 

 from my papers and from those of the most reliable embryo- 

 logists that at this stage the same relation is maintained 

 between the undivided nucleated periblast and the yolk which 

 it envelops. These portions of the ovum together represent 

 and are homologous with, as I said in the paper referred to, 

 the nucleated yolk-cells of the frog's ovum. 



M'Intosh and Prince deny the correctness of Agassiz and 

 Whitman's statement that there is a definite separation during 

 the later stages of segmentation between the central part of 

 the blastoderm and the subblastodermic periblast. They say 

 that the line of demarcation is broken in sections by knob-like 

 processes which project from the blastoderm into the yolk ; 

 but the figure they refer to in support of this statement does 

 not include the yolk at all ! In fact the figures they give of 

 sections of the blastoderm are quite unsatisfactory, and seem 

 to indicate that the sections themselves were too imperfect to 

 prove anything. At any rate my own sections of the ova of 

 the mackerel at different stages of segmentation entirely 

 confirm the results of Agassiz and Whitman so far as concerns 

 the complete demarcation between the central cells of the 

 blastoderm and the subblastodermic periblast. I differ, 

 however, from those authors in denying that there is actually 

 any space beneath the blastoderm in the living ovum ; the 

 suiface of the subblastodermic layer, though distinct from, is 

 in contact with, the lower surface of the blastodermic cells. 



1 fully agree with M'Intosh and Prince when they state 

 that a cavity appears between the under surface of the central 

 portion of the blastoderm and the periblast after the stage of 

 simple segmentation is completed. But they say, " We speak 

 of it as a germinal cavity, and do so advisedly, for it is not 

 the cavity of Von Baer, better known as the blastoccel or 

 segmentation-cavity." It will probably be difficult to con- 

 vince these authors that this cavity is, as Balfour and the 



