102 DE. WILSON POX ON THE DEVELOPMENT 



600 diam. linear. I have, however, found that it is best to use a power of 900 for this 

 purpose, Powell and Lealand's -j^ object-glass with eyepiece No. 2 giving a much clearer 

 view of many points than can be obtained with lower powers. 



' For the proper investigation of the earliest stages in the Chick and Mammalia I regard 

 a power of at least 900 as essential, and many points can only be satisfactorily elucidated 

 with a power of 1250 or 1850 diam. lin. 1 have used for this purpose a -^ object-glass 

 of Powell and Lealand's, respecting the value of which I can endorse all that has been 

 said in its favour by Dr. Lionel Beale. 



In the Tadpole, immediately after quitting the egg, there will be found at the extre- 

 mity of the tail muscles in all stages of development. These are represented in Plate V. 

 figs. 1 to 9. 



The earliest forms which indicate any differentiation from the round cells of the 

 embryo are indicated by the appearance of oval bodies measuring from -5^ to -g-^ of 

 an inch in length with a breadth of tooo *« tmTo- They contain a clear oval nucleus, 

 measuring -g-^xTo X Wo of an inch in its longer and shorter diameters respectively. The 

 remainder of this body is densely filled with black pigment-granules and glistening 

 scales and masses, regarding which I have no further observations to offer. 



I am disposed to term these bodies cells, not that I have been able to see around them 

 a well-defined membrane, such a structure not becoming apparent until a somewhat later 

 stage of their development ; but their outline is so sharply defined, they alter their shape 

 so little under moderate pressure, and form such distinct isolated anatomical elements, 

 that I believe that a wall must exist around them even at this earliest stage, especially 

 as one can be proved to exist at a period very little later in their development, and to 

 which the transition only takes place by insensible gradations*. As this, however, is still 

 a subject of considerable discussion among anatomists f, I can only give these reasons for 

 my opinion with considerable diffidence. I have never observed any earlier stages of 

 these bodies, nor any appearances of the building up of granular matter around a 

 nucleus, and I believe them to result from the first differentiation of the round formative 

 cells of the embryo, — bodies around which there is an almost equal difficulty in proving 

 the existence of a cell-wall, but which maintain their individuality and uniformity of 

 size and structure both under the condition of mutual pressure, and also when artificially 

 separated. These structures, which with the above explanation I shall, for convenience 

 sake, call cells, then elongate, so as to attain a considerable length without any necessary 

 alteration in the apparent size of the nucleus, which ordinarily maintains a central 

 position in relation to the long diameter of the cell ; but laterally, as seen in profile, it 



* On reexamining some of my preparations after preservation for twelve months in strong glycerine, I find 

 that in some of these early cells the contents have shrunk, and that a membrane has become quite distinct 

 around considerable portions of their outline.- — June 6, 1866. 



t See especially M. Schulze, " TJeber Muskelkorperchen und das was man eine ZeUe zu nennen habe," 

 Ueichert und D. B. Retmond's Archiv, 1861. 



