xvii PEOTOCHOKDATA 613 



round the egg towards the animal pole. The yolk is thus displaced 

 from this pole, which, although it was temporarily the seat of the 

 most of the yolk, remains ever afterwards relatively free from, yolk 

 and becomes the seat of an accumulation of clear cytoplasm ; although 

 even here a thin external layer of cytoplasm, with yellow yolk- 

 grains, is found. 



At the next cleavage the two cells become four ; the two anterior 

 cells are slightly larger than the two posterior, which receive all the 

 yellow substance. The four cells then become eight in the usual 

 way, by dividing into two tiers, and the four cells at the animal pole 

 are smaller than the four at the vegetable and contain all the clear 

 cytoplasm. There is a cross furrow observable when eggs in this 

 stage are viewed from the side. This arises from the circumstance 

 that the furrows separating upper and lower cells come to slant 

 downwards and forwards, and consequently they strike the vertical 

 furrow dividing anterior from posterior cells in different places. 

 The anterior vegetative cells are the largest, and the anterior animal 

 cells the smallest in the egg. 



We may here anticipate the final result of the development so 

 far as to say, that the animal pole of the egg corresponds roughly 

 to the antero-ventral region of the embryo, whilst the vegetative 

 pole corresponds to the postero-dorsal region of the embryo. If the 

 reader confuses " animal " and " dorsal " regions of the egg, it will 

 be impossible to follow the account intelligently. 



When the next cleavage occurs the spindles of all the dividing 

 nuclei are parallel to the horizontal plane, but are oblique to the 

 vertical plane of the egg. The 16-cell stage (Fig. 443, C) consists 

 of two tiers of eight cells. The animal hemisphere, in consequence 

 of the direction of the spindles of the dividing cells, is narrow in 

 front and broad behind, whereas the vegetative hemisphere is broad 

 in front and narrow behind, and here the two smallest cells in the 

 egg are delimited ; these consist almost entirely of yellow substance 

 and constitute a valuable landmark. The two cells immediately in 

 front of these contain the rest of the yellow substance, but in 

 addition a great deal of yolk. The remaining four of the vegetative 

 cells contain principally the slaty-blue yolk, but have a certain 

 amount of clear substance near their anterior borders. 



The next cleavage period raises the number of cells to thirty-two. 

 On the vegetative side of the egg the purely yellow cells divide so 

 as to form four, lying side by side. The half-yellow, half-blue cells 

 divide so as to separate the yellow and Hue portions ; and the yellow 

 portions lie to the outside, so that there is now a crescent, concave 

 in front, of six yellow cells, running across this hemisphere. The 

 anterior, wholly blue cells, of the vegetative hemisphere, divide into 

 anterior and posterior daughters ; the anterior daughters lying lower 

 than their sisters. There result two crescents of four cells each, 

 the front one lying just beneath the equator of the egg, and the 

 posterior running across the middle of the back. The cells of the 



