MEMOIRS OP TUE NATIONAL ACADEMY OF SCIENCES. 431 



view tbat tbu iiuclciir fragiuent^s persist for a while iu the circulatiou as the blood plates, and 

 considers it probable that the latter take some part iu foriniug the paraglobuliu of the blood. If 

 the blood plate is then a degenerate body, it may be compared to the s[)ore-like masses of chro- 

 matin, which are discharged from the disrnpted cells iu the lobster or crayfish embryo. 



VII.— TUE OlllGIN AND UlSTORY OF WANDKUING OKLLS IN ALPHKtJS. 



The wandering cells iu 2\.lpheiis have a triple origin, from the blastosphere, from the invagina- 

 tion, and fron» 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 invagina- 

 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 int« the yolk (Pis. xxxii-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 cau not be distinguished after a 

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

 the thoracic-abdomiual plate, and the parts of the embryo iu front of it as wandering 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 ju'esent 

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

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

 of the egg from those of the wandering cells. 



The object immediately iu 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 beiug 

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

 uectiou with the thoracic-abdominal jtlate and have entered the yolk are enumerated as wandering 

 cells. In an earlier part of this paper I gave au account of the origin and suj)posed fate of the 

 wandering cells, the general conclusion beiug that iu the early stages (Stages iii-v) they pass 

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

 mesoblastic 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 ecpial probability as originating, in some 

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

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

 would not sui)port this idea, but the objection could not be satisfactorily auswered, aud neither 

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

 wandering ceils in Alpheus, aud 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 beeu enumerated in five different stages, including 

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

 to the early egg nauj)liufe 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 aud embryonic 

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



' There is a, certain convenience in thus referring to the embryo proper aud to the less difierentiateil regions, while 

 it is understood that all the cells constitnto the embryo. 



