No. I.] GERM-LAYERS IN CLEPSTiVE. m 



the epidermal stratum of the ectoderm, and it is possible that they 

 are directly employed in the formation of more important parts 

 of the head. Smaller micromeres are gradually added round 

 the primary quarterfoil, in the proliferation of which each of the 

 macromeres appears to share. A disc of small cells is thus 

 formed in which it is impossible to trace the genetic history of 

 individual elements. 



Second Period, ending with the Formation of Proliferating 

 Blastomeres. — All regularity of cleavage ends with the eight- 

 cell stage. Henceforth several distinct forms of cleavage will 

 be carried on simultaneously, each restricted to special areas or 

 blastomeres. Although there is scarcely anything in the exter- 

 nal appearance of the eight-cell stage to indicate the relation of 

 its parts to the future embryo, yet we know by what follows that 

 an immense work has already been accomplished. All those 

 fundamental conditions and relations implied in the terms ante- 

 rior and posterior, right and left, dorsal and ventral, are now 

 definitely established. The ground-plan of the future structure 

 is there, and the segregation and distribution of the building 

 material have advanced far toward completion. 



The second period concludes the finishing strokes of cleavage, 

 carrying us from the eight-cell stage to that in which the pro- 

 liferation of the germ-bands begins. The prospective character 

 of the work becomes more and more manifest, and architectural 

 forecasts begin to reveal themselves. It is a period of preparation 

 in which everything is ordered and appointed for ihe formative 

 work with which the third or embryonic period begins. Proliferat- 

 ing blastomeres are created, grouped, and stationed according to 

 the special kinds of work which they, as the artisans of the third 

 period, are destined to accomplish. There are exactly thirteen 

 of these blastomeres, symmetrically arranged in three primary 

 groups. One group, consisting oi three entoblasts, is represented 

 by the macromeres, a, b, and e, or rather, will be represented by 

 them at the end of this period, after they have ceased to con- 

 tribute to the ectoblastic disc. The second and third groups, 

 consisting respectively of two mesoblasts and eight ectoblasts, are 

 yet to be developed from the large posterior macromere, x. 

 The macromeres a, b, and c take no further part in the cleavage, 

 if we except the budding off of ectoblastic micromeres at the 

 animal pole, which does not sensibly diminish their size or alter 



