434 



MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



that the primary yolk cells are formed iu this way rather thau by emigratiou, 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 tritle too old. Iu Hoinarus the primary yolk cells arise by 

 delaiuination, as I have already shown iu a preliminary paper in 

 the develoi)meiit of this form (23, Fig. 5). 



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 1) 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 iugrowth of certain cells at the surface. The his- 

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

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

 all the primary yolk nuclei present. The plaue of the paper (sup- 

 posing the drawing to represent a sphere) nearly passes through 

 the point of invagination (in.). 

 Iu order to test the accuracy of the method, two eggs of this stage were studied [a, ii and b, 

 tl ot Table i), and 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 and 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 8 are constructed from the egg seen in Fig. (ii, «, of Table i). Figs. 10 and 

 9 represent corresponding curves constructed from the second egg (ii, h). The two sets of curves 

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



Fig. 6. — Diagram of egg iu invagination 

 .stage, constructed from serial sections, to 

 show all the primary yolk nuclei present. 

 For details of this egg, see Tahle I (II, rt). 

 Invagination. /7i=^point'of invagination, 

 nearly iu the plane of the paper. 



Fm. 7 — Curve constructed from serial sections, showing the distribution of the iirimary yolk nuclei iu the egg represented by Fig. 6. 

 (See Table i, Invagination stage, ii, a. ) 



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

 rt, II, twenty-two are in the ventral (?) hemisphere of the egg, and tifteeu in the dorsal. In egg 6, 

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

 nuclei iucliue toward the ventral side of the egg, and hence, as already inferred, are probably 

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



The curves showing the relations of the embryonic nuclei (Figs. 8 and 10) read from eud 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 

 {Ah. P.), near the center of which is the point of invagination. In fit)nt of this there is a more 

 extensive, but less depressed portion, corresponding to the embi^youic area [E. A.). The number 

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



