MEMOIRS. OF THE NATIONAL ACADEMY OF SCIENCES. 437 



Table i ssliows tlie reiuaikahlf. (act that the percentage of increase of wandering cells, which 

 in Stage iii was twice as great as that of other embryonic nuclei, has now dropped until it is actually 

 less than the hitter. This means that the wandering ceils are mnltiidyingless rajiidly or that their 

 numbers are being depleted. Tiie relative increase of yolk nuclei from Stage li to in, and from 

 Stage m to iv, namely, 81 percent and 17 per cent (showing a marked falling off), is not specially 

 significant, since these nunduTs depend largely upon the age of the embryo or time which elapses 

 between successive stages. But this element does not enter into the relation which exists between 

 the relative increase of wandering and embryonic cells in the same egg, expressed by SI per cent 

 and 41 per cent, respectively, in Stage iii an<l by 17 per cent and 2(1 per cent in Stage iv. 



The curve constructed from Stage iv (Fig. 11) shows the wandering cells much more widely 

 diffused through the egg than at any earlier period. The thoracic-abdominal plate is no longer 

 sharply marked off from adjacent parts, and a considerable number of nuclei underlie the optic 

 disks. 



In this egg there are thirty-seven nuclei in various i)liases of karyokinesis (Figs. 70, 75); two 

 belong to the yolk, three to the deeper cells of the thoracic-abdominal plate, and the rest to the 

 surface cells of the embryo. In the optic-disk region a very few cells appear to be delaminating; 

 the rest are cases of radial division. Cell disintegration has not yet become a disturbing factor in 

 the enumeration of cells. There are but two nuclei in the egg, one in the yolk, the other in the 

 deeper part of the ventral plate, which show any traces of dissolution. 



The above facts lead to the conclusion that the wandering cells are being rapidly depleted 

 at this time, that they leave the yolk in considerable numbers and attach themselves to tiie 

 growing embryo and to the extra-embryonic surface of the egg. They thus warrant our interpre- 

 tation of a cell like ye, Fig. 70, or J/f',Fig. 71,as a migrant from the yolk. While the wandering cells 

 are being rapidly depleted, it does not of course follow that they never receive any recruits from 

 any part of the egg without the limits of the ventral plate, and it requires no very elaborate calcu- 

 lation to show that wandering cells are making early coiitributiqns to the mesoblast, but the study 

 of individual sections aiul the facts brought out in Table i, prove beyond a doubt that the only 

 cells which enter the yolk, up to this time, arise from one of the three sources already named, the 

 blastoderm, the invagination, and the ventral plate. 



Staiie V. — Eog-Xaiipliitx. — The early egg-nauplius stage is represented by two individuals, 

 one (V. a. Table i) cut in transverse and tiie other (v. h) in longitudinal vertical i>lanes. Kesi)ect- 

 ing the wandering cells we now notice: (1) that their numbers have markedly decreased; (2) that 

 they are far more widely and evenly distributed; and (3) that many are close upon or in contact 

 with the embryo or with the general surface of the egg. 



In egg V, b (Table i) the number of yolk cells is oidy one hundred and twenty eight, consid- 

 erably le.ss than are present in Stage ill, a decrease of 87 per cent, while in the other egg the 

 decrease is 21 per cent. On the other han<l, the rate of increase of embryonic nuclei is greater 

 than at any previous stage, .")'.) i)er cent in one egg and 6!) per cent in the other. That this is not 

 explained by a large interval of time existing between Stages iv and V is shown by the fact that 

 during the period (Stage iv aiuI Stage v, h, Table i) the total number of nuclei of the egg has 

 scarcely more than doubled, while the iiercenlagc of increase of embryonic nuclei has more than 

 trebled. Between Stages ii, a, and iii, on the other hand, the number of nuclei has nearly doubled, 

 while the i)er(^entage of increase of embryonic nuclei has risen from .'?2i to oidy 42. 



How is this very rapid increase in embryonic lUK^lei and coiirdinate decrease in wandering 

 cells explained in the egg-nauplius stage? The conclusion reached in Stage IV ajjplies here also, 

 with a certain restriction. The problem is now not a simple one, sin(;e perturbations caused by 

 the disintegration of nuclei ajipear to some extent in this stage. The diminution in ilie number of 

 wandering cells is now due to two causes, to cell disintegration and to the gradual subtraction of 

 cells from the yolk by cmigraticm. Disintegration of cells occurs both in the yolk and in i)arts of 

 the embryo. It is i)erhai)S most marked near the line of contact of the yolk with the embryo. Yet 

 the embryonic cells are meantime making a rapid net gain. 



We are thus warranted in our conclusion that the wandering cells, whiclj spread far and wide 

 through the egg, play a formative role in development, to a large extent at least. This conclusion 

 is rendered certain by the changes which ensue between Stages ii and iv, already noticed. The 



