MEMOIRS OF THE NATIONAL ACADEMY CF SCIENCES. 



433 



STAGE II. Close of yolk segmentation Formation of yolk cells, followed by imagination. A 

 surface view of this egg is given in Fig. 47, PI. xxx. The curve (Fig. 5 of text) shows that the 

 blastodermic cells are distributed very uniformly. In other words 

 the embryonic area is not as yet marked oft'. The distribution of 

 the primary yolk nuclei, of which there are exactly thirty-four, is 

 shown in the constructed figure (Fig. 3), which represents the egg 

 as it would appear if the yolk were transparent and the nuclei 

 opaque. The distribution of these nuclei through the egg is given 

 more completely by the curve (Fig. 4). The only questions which 

 need detain us in this stage are, how do the primary yolk cells 

 arise and from what part of the surface do they come ? Karyo- 

 kinetic figures, which abound among the surface nuclei, ought to 

 furnish au immediate answer to the first question. In this egg no 

 less than sixteen nuclei are met in various phases of division, fif- 

 teen of these belonging to the superficies and one to the central FIG. 3. Diagram ..r,-^ in ,i,hn,,m;iti,.n 



, . . sta^i- constructed from serial sections, 



portion of the yolk. Clusters of two and rarely ot three nuclei also ^.^ . iU tl)t . iirim . (ry yiilk , ,.,, s F , ir 

 occur at the surface, showing that cell division is active. In every details, * Tabi,. 1. st^e n (iMami.m. 

 case the cleavage is radial or perpendicular to the surface, and in no 

 instance have I seen au unambiguous case of delamination (v. PI. xxx). It is possible, however, 



FIG. 4,-Curve construed from serial sections, slum-in- tlio distrilmtiun of tin- primary Yolk nuclei in tbe ens represented l,y Fiji. ' 

 For further details, compare Table i, Stage II (Delamination). ,i = Anterior; 1'= Posterior. 



;A//V< 



H-H-h 



: 



FIG. 5._Cnrve showins tbe distribution of nuclei ,,t tlie surface ttbat is. nuclei of the embryonic cells, exclusive uf primary yolk . .11-, 

 of the egg represented by Figs. 3 and 4. For details, sec Table l. Stajze II (Delamination ). 



S. Mis. 94- 28 



