MEMOIRS OK THE NATIONAL ACADEMY OF SCIENCES. 439 



and the embryonic area is included between sections 14 and 41. The marked peripheral distri- 

 bution of the migratory cells is very significant. There seems to be a general movement of these 

 bodies to all parts of the supertic.ies. 



What is the ultimate fate of those cells which wander out to the surface of the egg? Fig. 34, 

 PI. XXIX, represents part of a. sect ion of the extra-embryonic surface at this stage. Here is undoubted 

 ectoblast (Ep.). and cells (Y. C.), 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 proximity to the surface. Do these cells (Fig. 34, Y. C.) eventually contrib- 

 ute to the mesoblast or ectoblast of the embryo? This question can not 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 liniug 

 of the midgnt. 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 invaginate wander- 

 ing cells. 



In a lobster's egg at the delamination stage, equivalent to Stage II, Table I, I find 213 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 thaf 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 raesoblast 

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

 (54) to originate in 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 Alpheus 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 developmental 

 stages are passed very rapidly. At Woods Holl, Mass., the late egg-nauplius of the lobster, 

 Homar-ux americantis, is from fourteen to sixteen days old, while a similar stage is reached by 

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



VIII. THE DEVELOPMENT OF THE NERVOUS SYSTEM. 



The nervous system can be referred back in the embryo to an early stage (Figs. 58, 62, 68), 

 when V-shaped ectoblastic thickenings unite the optic disks to the thoracic abdominal plate. The 

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

 dense sheet of ectoderm. There is an apparent concrescence of the limbs of the V, and in the egg- 

 uauplius (Pis. XLI, XLII) these thickenings form a pair of more or less closely united cords, which 

 are separated on the middle line'by a median longitudinal furrow. The shallow furrow is formed 

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

 sophageal ganglia to the segment of the first maxilhc. 



The nervous system of the egg-nauplius is not differentiated from the general integument, and 

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

 the base of the first pair of auteuiue (Fig. 116) it has the appearance of an elliptical plate in trans- 

 verse section. 



* Weldou, whose paper 011 the germinal layers in Crangon has been referred to, Bays truly that the difference 

 between iuvaginated cells is not Buflieieut to enable one, to say that certain cells are endoderm and that others are 

 mesoderni, but be designates as eudoderui all cells which are derived from the iuvagiuatiou, and restricts the origin 

 of the mesoderni to the lower layer cells of the ventral plate. Judging from the evidence which has thus 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 be interpreted as mesoblast. The thoracic-abdominal thickening is composed of a pair of concave 

 "neuro-muscular" or ventral plates, which correspond to tbe single plate described in Alpheus. 



