KARYOKIXESIS. 31 



four cells thus formed (A, B, C and D,) equal in size, but they each contain about 

 the same quautitj- of j'olk. Two of the cells (B and D) meet at the vegetal pole in 

 a polar furrow, whereas all four cells .usually meet in a point at the animal pole. 



From these four cells thus formed three groups or "quartettes" of small cells, 

 without yolk, are cut oif (Plate VI, figs. 89-96). These three quartettes (la-Id, 

 2a-2d, 3a-3d) form the whole of the ectoblast of the embryo. The fourth quartette 

 (4a-4d) consists of large cells containing yolk and one of the cells of this (juartette 

 (4d) is the mesentoblast and gives rise to most of the mesoblast and also to the 

 posterior part of the intestine. The other three cells of this quartette (4a, 4b, 

 4c) are purely entoblastie. The first division of the first quartette (la-Id) is very 

 unequal, giving i-ise to four large "cephaloblasts" and four small ••trochoblasts" 

 (figs. 93-96). The latter are peculiar in structure and history, being clear and non- 

 granular as compared with the cephaloblasts ; they divide but once and grow to a 

 great size, giving rise to parts of the velum and head vesicle. The first subdivision 

 of the cephaloblasts is also unequal (figs. 97, 98), giving rise to four small "apical 

 cells" and four large peripheral ones which become the ''basal cells" in the arms of 

 a cross of ectoblastic cells, which lies with its center at the apical pole and one arm 

 in each quadrant (figs. 99, 100). The first division of the second quartette is nearly- 

 equal (figs. 96, 97). while at the second division four small cells arise which forms 

 the '"tip cells" in the arms of the cross (figs. 96-100). 



Now as contrasted with these external phenomena of cleavage, which are 

 chiefly concerned with cell boundaries, the internal phenomena consist of certain 

 cyclical changes in the nucleus, centrosome and cj^toplasm, each c3Tle being in the 

 main like every other, though often differing in details. It is in these internal phe- 

 nomena that the causes of determinate cleavage must be sought and to a study of 

 these phenomena we now turn. 



No sharp line of demarkation can be drawn between the fertilization and the 

 first cleavage, since the two overlap, to a certain extent, in point of time. For con- 

 venience, however, we may consider the fertilization ended and the first cleavage 

 begun when the centrosomes have taken their definitive positions at the poles of the 

 incipient mitotic spindle. Such a stage is shown in figs. 54-55. 



1. The Nuclear Changes During Cleavage. — a. Independence of Germ 

 Nuclei} — Until the metaphase of the first cleavage the chromosomes derived from 

 the two germ nuclei are plainly separated into two groups, one derived from the 

 egg nucleus, the other from the sperm, figs. 55-56, tex:t fig. V. During tliemetaki- 

 nesis no such separation is recognizable, but in the late anaphase the chromosomal 

 vesicles fuse together into two groups and as the daughter nuclei become vesicular a 

 partition wall is left between these groups, fig. 00, and text lig. VI. In the telo- 

 phase this partition wall gradually disappears, persisting longest on the side of 

 tlio nucleus next the centrosome, where a groove marks its position, fig. 81; 

 this groove usually disappears at the height of the nuclear "rest" or ''pause," but 

 it appears again in the early propha.'ie of the ne.xt division and in almost exactly 



' An abstract of this section appeare<l in tlie Bioloijical Bulletin, Vol. II, UlUl. 



