144 PROCEEDINGS OP THE AMERICAN ACADEMY 



cells of the nerves, for instance, simply oval, as the first step to elonga- 

 tion ; next they are in rows ; then the ends in contact are without walls, 

 so that each cell opens into its neighbor ; and finally, all trace of the 

 separate cell-wall is lost in the straight sides of the nerve tubule, with 

 nothing but the mesoblasts to indicate the original position of the cells. 

 In the chorda dorsalis, intestines, vertebrae, muscles, &c., similar and 

 apparently gradual changes have been observed ; but each step, in most 

 instances, was investigated isolately from the previous one, and the 

 intervening space bridged over by the process of inductive reasoning 

 alone. This is not enough ; now we know that every second of the 

 life of a cell, or series of cells, may be traced most minutely, minute by 

 minute, hour by hour, and day by day. Day and night, watches have 

 been kept by observers in other depai'tments of science, and why may 

 not the naturalist do so? In some cases a very extensive series of 

 changes may be observed in a short time ; for instance, in the embryo of 

 the common Bream (Pomo<^s vifZ^ans), which Professor "Wyman has ob- 

 served to pass from the segmenting of the yolk to hatching in the space of 

 about forty hours. It is not possible, in any way, to trace the gradual 

 metamorphoses of cells and organs, except upon the living body ; other- 

 wise, every observation is a record of a detached fact, and no more ; 

 eveiy bit of an organ is subjected to all sorts of manipulations to bring 

 out what too often is not there according to the laws of the living being. 

 Reagents at one time, and pressure at another, reveal, not the truths of 

 nature, but our carelessness and presumption. I have in mind a re- 

 markable instance of the evils of the almost monomaniacal habit of 

 using pressure whilst investigating tissues. A celebi'ated physiologist, 

 in all probability, missed the most fortunate chance of discovering the 

 key to the whole history of the mode of origin of the embryo from the 

 yolk-cells, simply by using a bit of thin glass to cover the object on his 

 glass slide. Just before the segmentation of the yolk, the full-grown 

 yolk-cells of birds, turtles, if not all scaly reptiles, and sharks, are 

 very thin-waUed, hyaUne, globular vesicles, each one of which contains 

 a more or less darkened mesoblast, and within the latter are a certain 

 number of entoblasts (nucleoli). Now under the least pressure, the 

 cell-wall bursts quickly, and the mesoblast becomes fissured or wrin- 

 kled. In this condition the mesoblast was figured and described as the 

 yolk -cell proper, by no less careful an observer than Johannes Muller. 

 Now in the turtle, at least, the mesoblast undergoes self-division until 



