436 



MEMOIES OF THE NATIONAL ACADEMY OF SCIENCES. 



Table II, 



Of tbe forty-eight to sixty cells which appear hehiw in surface at this time in the ingrowing 

 cell mass, a large number (twenty in b, ii) are [)artiiig coiii[tany witii their lellows and beginning to 

 migrate into the yolk. So that at this stage the primary yolk cells receive their first recruits from 

 the ventral plate. 



Stage III. — Optic disks and thoracic-abdominal or ventral plate. — Wamlering cells now become 

 a very marked characteristic of the Alpheus egg (Pis. xxxii, xxxiii). We see by Table i that the 

 total number of yolk nuclei has come up to 199, an increase of 81 per cent against an increase of 

 only 4li per cent on the part of the othf^r embryonic cells. In other words, the wandering cells have 

 increased nearly twice as rapidly as the other nuclei of the egg. It is, perhaps, hardly necessary 

 to point out that this rapid gain is due to at least four possible causes : (1) to the multiplication of 

 primary yolk cells, probably the least important; (2) to the irruption into the yolk of invagiuate 

 cells, or cells derived from these, the most important source; and (3) to the multiplication of the 

 latter in the yolk itself; and (4) to migration from the ventral plate, which is formed by a thick- 

 ening about the point of invagination. 



The curve showing the distribution of wandering cells at this stage (which is not figured) is 

 nearly bilaterally symmetrical. The greatest depression is in the region of the thoracic-abdominal 

 plate, while on either side of this there is a marked drop in the curve, answering to nuclei which 

 underlie the optic disks and tbe parts behind them. (Compare Fig. 11 of text.) 



A considerable number of cells have migrated to points near the surface both behind the 

 ventral plate, to either side of it, and immediately in front of it (see Figs. 56, 58, 59, 60). A very 

 few have wandered out to points just beneath the optic disks. Quite a number have started in the 

 direction of the dorsal surface of the egg, but none have reached it. 



Stage IV. — Rudlmmts of First Antenncc and Mandibles. — Tliis is the most interesting stage 

 in some respects (see Pis. xxxiv, xxxv), since it is critical so far as the fate of the wandering cells 

 is concerned. 



OD. 



Abp. OD. 



Fig. 11, — Curve showiii:; tlic distribution of Avaiuli-rinj; lells in Stneo TV. mdimi^nt.'* nf lir-^t nntcnnrn and niandibU'S present. 

 PI. xxxiv. and Jor numerical dctail.s. sec Table i, .Stage iv.) Abp. region ui" v.-utral plate; O />. Kcgion of oi.Iir discs. 



{Compare 



