MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 439 



and the embryonic area is iiutliuled liotweon sections 1-t and 11. The marked peripheral distri- 

 bution of tlie niigratoiy cells is very significant. There seems to be a peneral movemeut of these 

 bodies to all parts of the superlicies. 



What is the ultimate fate of those cells which wander out to the surface of the egg? Fig. 'M, 

 VI. XXIX, represents part ol a sec^tion of theextraenibiyonic snrfac-e at this stage. Here is undoubted 

 ectoblast (Ep.), aiul cells (V. V.), which have undoubtedly come from the yolk, are pressing against 

 the surface. In a single egg one may meet with twenty or more such cells, as well as with cells 

 in various degrees of ]iroxiinity to the surface. Do these cells (Fig. .'>4, Y. C) eventually (contrib- 

 ute to the mesoblast or ectoblast of the embryo? This question can uot be answered by direct 

 observation, since it is clearly impossible to follow the fate of the individual cells. From all the 

 evidence which I have gathered I conclude that the wandering cells give rise to mesoblast — muscles 

 of body wall, connective tissue, and blood corpuscles, and, later, to the definitive entoblast lining 

 of the midgut. The part which the primary yolk cells play can not be decided, nor can it be deter- 

 mined whether degeneration is more characteristic of these elements than of the invagiuate wander- 

 ing cells. 



In a lobster's egg at the delamination stage, equivalent to Stage il. Table I, I fiud 21.'5 nuclei 

 present. Eleven of these belong to the yolk. Upwards of twenty, two of which are yolk cells, are 

 in process of division. Of the eighteen superficial or peripheral cells which are in karyokinesis, 

 two are dividing horizontally or delaminating. In the lobster the primary yolk cells degenerate, 

 in part if not wholly, at a very early stage, as already stated. 



In the egg-nauplius phase it was noticed that wandering cells settle down upon the ectoblastic 

 bands out of which the nervous system is subsequently developed (Fig. 127. Y. C). In this case 

 also it is impossible to ascertain with certainty the fate of such cells. They might form ectoblast 

 or mesoblast, but it is more probable that they contribute to the latter Layer only. 



It is evident that the history of the wandering cells is largely the history of the mesoblast 

 and entoblast. The mesoblast,* where it has been studied in Decapods, as in Astacus, is found 

 (54) to originate iu certain swollen cells in or near the anterior margin of the "blastopore" or 

 pit. From this primary mesoderm cells are budded off, which extend forward in a more or less 

 continuous sheet over the ectoderm. It is possible that in Alj)heus the wandering cells may serve 

 as a means of a precocious development of the mesoblast and entoblast. In connection with this 

 idea, it is interesting to recall the fact that in high temperatures of the tropics the develoiunental 

 .stages are passed very rapidly. At Woods Holl, Mass., the late eggnauplius of the lobster, 

 Homani.s americanux, is from fourteen to sixteen days old, while a similar stage is reached by 

 Alpheus saulcyi at Nassau, N. P., in about seven days. 



VIII.— THE DEVELOPMENT OF THE NERVOUS SYSTEM. 



The nervous system can be referred back in the end)ryo to an early stage (Figs. .58, 62, G8), 

 when V-shaped ectoblastic thickenings unite theoi>tic disks to the thoracic abdominal plate. The 

 intervening space is gradually encroached upon until the optic disks are completely bridged by a 

 dense shet't of ectoderm. There is an apparent concjresceuce of the limbs of the V, and in the egg- 

 nauplius (IMs. XLI, XLII) these thickenings form a pair of more or less closely united cords, whiiili 

 are separated on the middle line by a median longitudinal furrow. The .shallow furrow is forme<l 

 by the swellings of ectoderm which correspond to the future ganglia, and extends from the supraoe- 

 sophageal ganglia to the segment of the first maxilhe. 



The nervous system of the egg-nau[)lius is not dirterentiated from the general integument, and 

 the ectoderm is still a single layer on the niiddle line in the maxillary region (Fig. 121), while at 

 the base of the first pair of antenna- (Fig. IIG) it has the appearance of an elliptical i)late in trans- 

 verse section. 



* Welilou, xvliOHe ii;i|ier ou tin- gtTiiiiiiiil layers in Cr.iiigoii li.is l)eeti referred to, says truly tliat tlie (liU'ereiice 

 between inv,ai;inate(l cpUh in not sufficient to enal>le one to say that ecrtaiu cells are eudoderm and that others are 

 mesoderm, but he desiijnates as eudoderm all cells which are derived from the inv.igination, and restricts the origin 

 of the mesoderm to the lower layer cells of the ventral plate. Jndging from the evidence which has thns far been 

 presented, the cells which be bas marked endoderm, lying against the embryo and near the folds of the appendages, 

 are in my opinion to bo interpreted as mesoblast. The thoracic-abdominal thickening is composed of a pair of concave 

 "neuro-musciilar" or ventral plates, which correspond loathe single plate described in Alpheus. 



