MEMOIRS OF THE NATIONAL ACADEMY CF SCIENCES. 



433 



Stage ll.—Olose of yolk negmentatioti— Formation of yolk cells, followed by invagination.— A 

 siirftice view of tliis egg is given in Fig. 47, Tl. xxx. The curve (Fig. 5 of text) shows that the 

 blastotlermic colls are distributed very unifonnly. In other words 

 the embryonic area is not as yet marked ott". The distribution of 

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

 shown in the constructed ligure (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. i). The only qnestious 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 J Karyo- 

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

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

 less than sixteen" nuclei are met in various i>hases of division, fif- 

 teen of these belonging to the superficies and one to the central vu-.. 

 portion of the yolk. Clusters of two and rarely of three nuclei also "'•'*'"'• '^""'""'"'••' '"^""' "'"''' »'•««'>"». 



shuwtut; III! tlu) primary yolk **ells. For 



occur at the surfiice, sbowiug that cell division is active. In every .u-taii^, s^e Table i. sta-o n (Drian.ii.a. 



case the cleavage is radial or perpeudicular to tlie surface, and in no ****"* 



instance have I seen an unambiguous case of delamiuation (v. PL xxx), It^s possible, however, 



-Diagram of <•;;*» 



<U-laiuiiialiii 



Fig. 5. — Cnrv*; Hhowint; the (U.stribution of nuck'i at tin* Hiirfare (thai ih. niirlci of thu embryuuic cuUa. exclusive of priiuiuy yolk cells) 

 of the ogE rcprc8t*nt*'<l hv Fi;;^. W aud 4. For dotaila, aco Tabh- l. Sla;;*' ii (r>i?hiiiiiiiaiion}. 



S. Mis. 94- — 28 



