MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 403 



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

 cords lias quite closed over the central or sternal region of this part of the embryo (St. A.). Cell 

 outlines arc very distinct at the surface in preparations, and they are sometimes well defined in cells 

 which have passed from the surface to parts below it. in both the region of the optic disk and 

 that of the ventral plate (Fig. 80, EC.), but elements closely associated with yolk are usually amu'- 

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



The arrangement of the embryonic cells of the superficial epiblast in beautiful curves and rings 

 around definite, centers orthogouic systems of curves is not nearly so pronounced as in the embryo 

 crayfish (Astacusfluviatilis), according to the delineations of Keichi'iibach and Winter. Eeichen- 

 bach states that in the crayfish the superficial embryonic cells multiply 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 tips of plants. According to Sachs, Iteich- 

 eubach, and others, the cell nuclei always divide in 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 right angles to this, producing new strings. Thus there is developed 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 prevalence of such a law in the earlier stages. The early embryo of Alpheus is much less 

 diffuse than in the crayfish, and the different 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. 88-90), and is 

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

 I"). These sections give some interesting facts with reference to the role of the wandering cells. 

 The first (Fig. 88), which is nearly median, cuts the ventral plate and below it the cells which are 

 migrating from it into the yolk. A continuous layer of cells extends anteriorly to the transverse 

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

 superficial epiblast. 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- 

 pendages (A. I, A. II, Md.). The folds of the latter arise through the ingrowth of superficial cells. 

 Here another cell (Y. C. 1 ) is close to the outer surface of embryo; another (Y. C. 2 ) is in a distant 

 part of the egg and is in the aster stage of karyokinesis ; others still ( Y. C. 3 ) have wandered in 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. 

 93, C.M.), are met with, and one of them (Fig. 90, EC. dotted line extended) has just passed below it. 



Figs. 80 and 87 are parts of longitudinal sections of an embryo six hours older than the last. 

 The first exposes the optic disk (O. D.) ami 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- 

 blastic cells (Y. C.) have already attached themselves to it. (Compare 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 epiblast consists for the most part of a single layer of cells. Two large 

 wandering elements (Y. C. 1 ) 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 passes 

 obliquely 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 us beyond 

 a doubt what is the fate of large numbers of wandering cells, present at this time. As has been 

 already shown by preceding figures (see Figs. 73, 88, and others), the cell mass constituting the 

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

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



