434 



MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



that the primary yolk cells are formed iu this way rather than by emigration, and that my failure 



to detect the actual process is due to the fact that I did not sec- 

 tion exactly the right stage, the egg shown in Figs. 38-45 being 

 a trifle too old. In Homarus the primary yolk cells arise hv 

 delamination, as 1 have already shown in a preliminary paper iu 

 the development of this form (!'.'!, Fig. .">). 



Sections of this stage show conclusively that the primary 

 yolk nuclei do not come from any one point on the surface, but 

 that the majority of them may come from a restricted area of 

 the egg. In Fig. 3 about two-thirds of the nuclei present are con- 

 fined to the lower (ventral ?} surface of the egg. They are in 

 various degrees of progress from the surface toward the central 

 parts, which the majority have already reached. 



The formation of primary yolk cells is followed by the in 

 vagination and ingrowth of certain cells at the surface. The his- 

 tology of the embryo at this phase is given in PI. xxxi, and Fig. 

 (> (of text) is constructed from the entire series of sections to show 

 a " " ie primary yolk nuclei present. The plane of the paper (sup- 

 posing the drawing to represent, a sphere) nearly passes through 

 the point of invagination (in.). 

 In order to test the accuracy of the method, two eggs of this stage were studied (, n and b, 

 u ot Table i), aixl the results show a remarkable agreement. Thus there are exactly thirty-seven 

 primary'yolk nuclei in each egg, and the total difference in the number of embryonic nuclei in the 

 two eggs is only nine. Curves were constructed to show the number aud distribution of yolk nuclei 

 and embryonic nuclei in both eggs, and the two are introduced here because of the striking simi- 

 larity. Figs. 7 and S are constructed from the egg seen in Fig. G(n, n, of Table i). Figs. 10 and 

 '.) represent corresponding curves constructed from the second egg (u, /*). The two sets of curves 

 tell exactly the same story in each case, and it is not necessary to dwell upon it. 



I 1 '!'' (> - Pia^runi nl r-j-r in m\ ;i^in;il mi 

 st.'ijjr-, rmistriirtril i'roiu serial N.I linns, t. 

 sliow ;ill tin- jiriinary yolk Jiinl.-i pi. s. nl 



FOr (IftjIlN M|' lt,is |'M.r i S( ,,. -|-;,|l]f I (II. "I 



Cnvagiiiation. In ]''inl <(' in\ ;i^in;itioji 

 tii'iU'lx iii Llie j)l;iiH- nt' thr pa JUT. 



Fio. 7. Curve coDstruotcil from serial sections, sli 

 (See Table i, Invagination stajti-, u, a.i 



wiu^ (lie di.^trilmtion of tbe primary yolk nuclei iu. the e"" represented bv Fie 6 



The bulk of the primary yolk nuclei are placed near the center. Of the thirty-seven nuclei in 

 , ii, twenty-two are iu the ventral (?) hemisphere of the egg, aud fifteen in the dorsal. In egg ft, 

 II, twenty-one nuclei are situated in the ventral half and sixteen in the dorsal. Thus these yolk 

 nuclei incline toward the ventral side of the egg, aud hence, as already inferred, are probably 

 derived in great measure from that part which corresponds to the embryonic area. 



The curves showing the relations of the embryonic nuclei (Figs. 8 aud 10) read from end to 

 end of the embryo (posterior to anterior), the sections being transverse to the longitudinal axis. 

 The greatest depression naturally occurs in the region of the thoracic-abdominal or ventral plate 

 (Ab. P.), near the center of which is the point of iuvaginatiou. In front of this there is a more 

 extensive, but less depressed portion, corresponding to the embryonic area (E. A.). The number 

 of cells entering into the ventral plate at this time are shown in Table n. 



