MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 403 



crowded cells (T. Cd.), and the backward extension of this, and tlio approximation of tlie lateral 

 cords lias quite closed over tlie central or sternal region of this i)art of the eud>ryo (St. A.). Cell 

 (lutliiies an- very distinct at the surface in preparations, and they are sometimes well defined in cells 

 wiiich have passed from the surface to parts below it. in both tiie I'efiion of the optic disk and 

 that of the ventral plate (Fig. SO, Ec), but elements closely associated with yolk are usually am<e- 

 boid. The nucleus of the epiblastic or epithelial cell on the confines of the embryo, or on the extra- 

 embryonic surface of the egg, has the shape of a flattened, round, or oval disk. Epiblastic nuclei 

 in the appendages and other parts of the embryo, where there is rapid cell division, are angular 

 in conseipience of crowding, and deep-lying nuclei are generally spherical. 



The arrangement of the embryonic cells of the superficial ei)iblast in beautiful curves and rings 

 around dclinite centers — orthogonic systems of curves — is not nearly so pi'onoun(;cd as in theend)ryo 

 crayfish (A.stiicitsjiiiviatilin), according to the delineations of lieiclienbach and Winter. Keichen- 

 bach states that in the crayfish the 8ui»erficial embryonic cells multi[>ly about a given center, like 

 that of the "head fold" (optic disk), or " thoracic abdominal rudiment," according to definite 

 laws. This was discovered by Sachs in the growing tiiis of i>lants. According to S;u;hs, Iteich- 

 enbach, and others, the cell nuclei always divide iu one of two opposite planes; that is, they either 

 separate along a radius drawn from a given center, thus giving rise to radial strings of cells, 

 or in a plane at light angles to this, producing new strings. Thus there is developc<l about the 

 initial cell a series of concentric circles and radiating lines of cells. The positions of the equa- 

 torial plates of dividing nuclei, which some eggs of Alpheus show in abundance, do not indicate 

 the i)revalence of such a law iu the earlier stages. The early embryo of Alpheus is much less 

 diffuse than iu the crayfish, and the difierent cell groups soon impinge on each other, and their 

 relations are disturbed. 



Several transitional .stages between the last two embryos figured (Figs. 72 and 93) will now be 

 examined. The first is represented by three longitudinal sections (PI. xxxvi. Figs. SS-UO), and is 

 about seventy hours old. It is from the same jirawn as the segmented egg shown in PI. xxvii, Fig. 

 !;■>. Tlie.se sections give some interesting facts with reference to the role of the wandering cells. 

 TIk- first (Kii;. SS), which is nearly median, cuts the ventral plate and below it the cells which are 

 migrating I'rom it into the yolk. A continuous layer of c^lls extends anteriorly to the transverse 

 cord (T. C<1.). In this region a wandering, mesoblastic cell (Y. C.) is nearly in contact with the 

 siiptTficial cpiblast. The next .section touches the outer edge of the ventral plate (Fig. 89), 

 which is marked by large granular nuclei, and crosses the lateral cord and rudiments of the ap- 

 peiida^jes ( A. I, A. II, Md.). The folds of the latter arise through the ingrowth of sai>erficial cells. 

 Here another cell (Y. C) is close to the outer surface of embryo; another (Y. C.') is in a distant 

 part of the egg and is in the aster stage of karyokinesis ; others still ( Y. C'.^) have w^andered iu a 

 diametrically opposite direction. In Fig. 90 we see still more of these wandering cells, in this 

 instance, chiefly above the embryo. 



In the middle of the optic disk some of the large granular cells visible from the surface (Fig. 

 9.'i, CM.), are met with, and one of them (Fig. 90, Ec. dotted line extended) has Ju.stpa.ssed below it. 



Figs. SO and 87 are parts of longitudinal sections of an embryo six hours older than the last. 

 The first exposes the optic disk (O. D.) and gives evidence of the further increase of the latter by 

 the emigration of cells from the surface, and it is quite probable that .some of the wandering, meso- 

 bhustic cells (Y. C.) have already attached themselves to it. (Conqiare Fig. 90.) In the next (Fig. 

 87) the inner edge of the optic disk (O. D.) and the outer border of the ventral plate (Ab. P.) 

 are involved. Here the ei)il)last consists for the most part of a single layer of cells. Two large 

 wandering elements (Y. C) are in contact with the surface cells of the embryo in the neighbor- 

 hood of the appendages. 



Fig. 91 is from an embryo twelve hours older than the last. The plane of .section pa.s.ses 

 obliipiely through the optic disk, cutting the anterior half of one (to the left) and the posterior 

 part of the other. A study of this section and of the series to which it belongs, shows iw beyond 

 a doubt what is the fate of large numbers of wandering cells, [ire.sent at this time. -Vs has been 

 already shown by preceding figures (see Figs. 73, 8S, and others), the cell mass constituting the 

 thoracic-al)ilominal plate is now the principal source of the wandering yolk elements, and, as has 

 been al.so shown, they migrate into all parts of the egg, multiply by karyokinesis, and settle 



