MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 431 



view that the unclear fragments persist for a while iu the circulation as the blood plates, and 

 considers it probable that the latter take, some part in forming the paraglobulin of the blood. If 

 the blood plate is then a degenerate body, it may be compared to the spore-like masses of chro- 

 matin, which are discharged from the disrupted cells in the lobster or craytish embryo. 



VII. THE ORIGIN AND HISTORY OF WANDERING CELLS IN ALPHEUS. 



The wandering cells iu Alpheus have a triple origin, from the blastosphere, from the invagina- 

 tion, and from the thoracic-abdominal plate. Those which arise from the blastula at the close of 

 segmentation are, perhaps, the representatives of a primitive endoderm. Following the iuvagina- 

 tion, a thick pad of cells is formed, the ventral or thoracic-abdominal plate. From this plate a, 

 general migration of cells occurs on all sides into the yolk (Pis. xxxn-xxxv). While there is a 

 great tendency to migrate to parts underlying the embryonic area, the cells nevertheless wander 

 to all parts of the egg, even to those most remote from the embryo. The first of these wandering 

 bodies which originate from the blastosphere have been called "primary yolk cells." The latter 

 classes may be called "wandering cells." Since these classes can not be distinguished after a 

 certain period, I refer to all cells which move about iu the yolk and have no direct connection with 

 the thoracic-abdominal plate, and the parts of the embryo in front of it as wander ing cells. I have 

 been somewhat at a loss to find a suitable term for these bodies, since there are obvious objections 

 to the use of "yolk cells" or "yolk nucleus." Where these terms have been employed in the present 

 paper, they must be understood to refer to the wandering cells which have been defined above. 

 The term "embryonic nuclei" is used for convenience merely to discriminate the remaining nuclei 

 of the egg from those of the wandering cells. 



The object immediately in view is to determine the fate of these wandering cells, to ascertain 

 what formative role they play while the mesoderm, and more particularly the endoderm, are being 

 differentiated into definite cell layers. In the following account, cells which have parted all con- 

 nection with the thoracic-abdominal plate and have entered the yolk are enumerated as wandering 

 cells. In an earlier part of this paper I gave an account of the origin and supposed fate of the 

 wandering cells, the general conclusion being that iu the early stages (Stages m-V) they pass 

 from the yolk to the embryo and to the extra-embryonic parts* of the egg, and contribute to both 

 niesoblastic and entoblastic structures. 



A number of friends to whom I showed my sections objected to this interpretation on the 

 ground that these wandering cells could be regarded with equal probability as originating, in souie 

 measure at least, in the opposite way, that is, as budding from superficial cells not concerned with 

 the thoracic-abdominal plate, and migrating into the yolk. A careful study of successive stages 

 would not support this idea, but the objection could not be satisfactorily answered, and neither 

 view could be readily proved. I therefore undertook a renewed aud more precise study of the 

 wandering ceils iu Alpheus, and I think that their fate has been definitely settled. 



The number of wandering cells which occur iu the yolk, and the number of " embryonic cells" 

 (that is, all the other cells of the egg) have been enumerated in five different stages, including 

 seven different embryos, from the period of delauiination at the close of primary yolk segmentation 

 to the early egg-nauplius condition (Stages II-V.) This covers the most important period so far as 

 the wandering cells are concerned. The rate of increase of both wandering cells and embryonic 

 cells has also been determined for the successive stages, aud the data are given iu Table i. 



* There is a certain convenience in thus referring to the ombryo proper aud to the less differentiated regions, while 

 it is understood that all the cells constitute the embryo. 



