20 GENERAL ANATOMY OF THE TISSUES. 



§ 10. 



Free cell-development is, in man and the higher animals, 

 far less common than has been hitherto assumed, and under 

 this category we can enumerate, so far as is at present known, 

 only the development of the chyle and lymph corpuscles, of 

 the cells of certain glandular secretions (spermatic cells, ova), 

 and gland-like organs (closed follicles of the intestine, lymph 

 glands, splenic corpuscles and pulp, thymus); lastly, of the 

 cellular elements in the pregnant uterus, in the corpus luteiim, 

 in the medulla of foetal bones, and in the soft ossifying 

 blastemata. The separate steps of the process in this mode of 

 cell-development have as yet been traced principally in the 

 first-named cells, but much is yet wanting to complete our 

 knowledge of it. This much is certain, that the origin of the 

 cells is always preceded by the development of 

 cell-nuclei, while it is doubtful, on the other 

 hand, how these are formed. In the chyle and 

 in the spleen we see as the first indication of 

 cell-formation, rounded homogeneous-looking 

 corpuscles of 0001 — "002"', which, increasing somewhat in 

 size, soon clearly appear to be vesicles, and often, upon the 

 addition of water, exhibit in their interior, together with small 

 granules, a large granule, like a nucleolus. Whether this last, 

 as is certainly the case in the dependent mode of cell-develop- 

 ment, arises before the nucleus and is the condition of the deve- 

 lopment of the latter, or whether it is formed subsequently 

 therein; how, again, the nuclei themselves are developed, 

 whether as originally homogeneous corpuscles, which subse- 

 quently exhibit a differentiation into inner and outer parts, — 

 envelope and contents, or whether they are not, from the first, 

 vesicular, cannot at present be decided. 



The nuclei being once formed, the cell-membranes are 

 developed around them, though not always in the same way. 

 In the first place, they may be applied directly around the 

 nucleus, so that the nascent cell is but little larger than its 



Fig. 2. Contents of a Malpighian corpuscle of the ox, x 530 : a, small ; b, larger 

 cells ; e, free nuclei. 



