538 



SCIENCE 



[N. S. Vol. XLII. No. 1085 



plex vitelline circulation is fully established, 

 and all of the products of the yolli mesenchyme 

 completely difFerentiated. The study has been 

 greatly facilitated by a comparison of the 

 normal embryos with specimens in which the 

 circulation of the blood was experimentally 

 prevented from taking place. In such speci- 

 mens the cells on the yolk-sac never became 

 confused or contaminated with other cellular 

 elements introduced by the circulating blood. 

 The wandering cells may thus be completely 

 followed through all stages in their isolated 

 position. 



In the first place, I can not resist the im- 

 pulse to highly recommend that all students of 

 hematogenesis spend some time at least in a 

 study of living mesenchymal cells and their 

 histogenesis. Such a study will soon convince 

 one of the great disadvantages under which an 

 investigator labors in attempting to solve the 

 origin of blood from observations on dead mate- 

 rial in serial sections. The problem becomes 

 so simplified and devoid of laborious unin- 

 structive technique that it seems almost super- 

 ficial. One may learn as much from the living 

 yolk-sac in an hour of careful study as in al- 

 most a week's perusal of sections. Most im- 

 portant is the fact that certain things may 

 actually be seen to occur that sections could 

 scarcely stimulate the mind to imagine. The 

 only disadvanage is that the worker may be 

 led to wonder whether so apparently simple a 

 problem is actually of scientific importance. 

 Fortunately, this mental state is soon passed 

 over on realizing the necessary care and pre- 

 caution which must be taken in following the 

 movements and changes in the living cells. 



Each cell is to be recognized as a living 

 complex and the observer will realize the im- 

 portance as well as the difficulties of thor- 

 oughly understanding and interpreting cor- 

 rectly its manifold changes and behavior. 

 Material which to some extent allows such a 

 study is often available. The Fundulus yolk- 

 sac, however, is exceptionally adapted to this 

 study on account of the beautiful simplicity of 

 its structure, as well as the remarkable clear- 

 ness with which each cell may be observed. 



The results of this investigation of wander- 



ing mesenchymal cells may be summarized as 

 follows : 



The wandering cells begin to migrate away 

 from the embryonic shield or line of the em- 

 bryonic body at an early period, when the em- 

 bryo is about forty hours old, the germ ring 

 having almost completely passed over the yolk 

 sphere to enclose its vegetal pole. The cells 

 migrate away chiefly from the caudal end of 

 the embryo, only a few wandering out from 

 the head region. The regions of the yolk-sac 

 thus suggest an area opaca about the tail end 

 and an area pellucida around the neighborhood 

 of the head. 



All of the cells wander into the so-called 

 subgerminal cavity, the space Wilson^ and 

 others consider a late stage of the segmentation 

 cavity, between the yolk-sac ectoderm and the 

 periblast syncytium. 



When the cells first appear they are all 

 closely similar in shape and about the same 

 size. Very soon, however, they begin to ex- 

 hibit certain differences. Many become elon- 

 gate spindle cells with delicate filamentous 

 processes, sometimes producing a stellate ap- 

 pearance. Others are more ameboid in shape 

 with conical pseudopod-like processes which are 

 constantly being thrown out at one place and 

 withdrawn at another. StiU a third class of 

 cells appears somewhat later than the other 

 two; these are more circular in outline with 

 short pseudopods and are more slowly moving. 



The movements of these extremely numer- 

 ous cells and their changes of position may be 

 readily followed with a high magnification. In 

 embryos of about sixty hours, still some time 

 before the heart begins to beat or the blood to 

 flow, four clearly distinct types of cells can be 

 recognized among these originally similar 

 mesenchymal cells, and the further history of 

 the four types may be completely traced. 



The ameboid cells with conical pseudopod- 

 like processes shortly after sixty hours begin 

 to show an accumulation of pigment granules 

 within their cytoplasm. Just at this time 

 they are seen to be of two distinct varieties, 



1 Wilson, H. v., "The Embryology of the Sea 

 Bass (Serranus atrarius)," Bull. V. S. Fish Com., 

 1891. 



