78 MCMURRICH. [Vol. XL 



In the 32-celled stage a remarkable difference of appearance 

 supervenes, depending upon the cells having at length reached 

 the surface of the yolk and fused with the peripheral layer 

 (see Fig. 20), thus allowing a superficial indication of the seg- 

 mentation to appear. The eight ectoderm cells of the i6-celled 

 stage have given rise to sixteen cells (Fig. 13, Ec), which have 

 a characteristic appearance. Each has a more or less perfect 

 hexagonal outline in surface view, and in the center of the 

 area enclosed by cell-boundaries is the mass of protoplasm sur- 

 rounding the nucleus, which I have spoken of hitherto as the 

 cell. This mass of protoplasm is not, however, sufficiently 

 large to cover the entire surface of the cell, but sends off nu- 

 merous radiating processes which come into contact with those 

 of other cells, so that, even at this stage, when the true cell- 

 boundaries are distinguishable, the ovum is a syncytium, a 

 fact of which sections give ample evidence. The six dark 

 mes-endoderm cells of the i6-celled stage divide in such a man- 

 ner as to form a circle of twelve cells (Fig. 13, MEn), placed 

 somewhat obliquely to the antero-posterior axis, while the pos- 

 terior pole is occupied by four vitellophag cells (^z), which are 

 not, however, arranged exactly around the posterior pole, but 

 incline slightly towards the ventral surface. A marked differ- 

 ence in capacity for staining also becomes evident in the cells 

 composing the egg at this stage. The twelve mes-endoderm 

 cells, as the cells marked MEn in the figures may be termed, 

 are distinguished by the very deep tint which their protoplasm 

 shows, as well as by the apparent absence of processes radiat- 

 ing from it, while the vitellophag cells show, on surface views, 

 hardly any protoplasm at all, the darkly-stained nuclei standing 

 out prominently upon an almost unstained ground. The ecto- 

 derm cells are intermediate between these two extremes, their 

 protoplasm being distinctly visible, but assuming a much fainter 

 stain than the mes-endoderm. This peculiarity is in part due 

 to the different concentration of the protoplasm around the 

 nuclei in the three kinds of cells. As sections (Fig. 20) show, 

 the nuclei of the mes-endoderm cells [MEn) are surrounded by 

 a large quantity of protoplasm, from which slender processes 

 pass outwards to unite with a thin layer of protoplasm which 



