THE DEVELOPMENT OF LEPIDOSIREN PARADOXA. 29 



dermic, there nppears to me to lurk a confusion of ideas 

 between the two pairs of antithetical terms — ectoderm and 

 entoderm (or epiblast and hypoblast) , and micromeres and 

 macromeres (or animal cells and vegetable cells). The latter 

 pair of terms are purely descriptive, and may be applied to 

 blastomeres at once upon the evidence of an isolated observa- 

 tion. The former, on the other hand, are terms associated 

 with definite theory ; they are not to be applied on mere 

 observations of sizes and shapes of cells, but involve the fate 

 of the cells. It seems to me quite impossible to define a 

 layer as hypoblastic except by asking one or other of the 

 two questions : (1) Does it form the lining of an archenteric 

 cavity ? and (2) Does it become a certain part of the definitive 

 epithelial lining of the gut? And if during the early stages 

 of development a certain set of cells become invaginated 

 along a considerable extent of the archenteric roof, this 

 seems to me in itself amply sufficient reason for calling such 

 cells hypoblastic quite apart from what their special cha- 

 racters of size, shape, content, and so on may be. There is 

 no justification at all that I can see for calling the small fine 

 yolked cells towards the upper pole of the egg epiblast, and 

 on their extension in along the ai'cheuteric wall to found the 

 statement that "ectoderm becomes invaginated." Because 

 these cells behave as they do they ai'e not ectoderm, but 

 entoderm. 



Further, a main character upon which these cells along the 

 archenteric roof are relegated to the categoi-y of " ecto- 

 derm" or "animal cells" is the finely granular character of 

 their yolk. Size of contained yolk granules is a form of 

 evidence which must be used with the greatest caution, for 

 wherever metabolism is active there the large yolk granules 

 are broken down into fine granules to facilitate assimilation. 

 All the yolk is destined to be so broken down eventually, and 

 the fact of its having done so in some particular part of the 

 esrsr earlier than elsewhere seems to indicate merely that 

 metabolism is there more active. Consequently I can attach 

 little weight to statements on the morphological nature of 



