CELL. 



[ 130 ] 



CELL. 



teniata. In the case of the chyle and the 

 spleen, at tlie commencement of cell-forma- 

 tion, there occin- ronndish,apparentlv homo- 

 geneous bodies of 1-11000 to l-rjGOU" dia- 

 meter, which, increasing in size, soon appear 

 distinctly as vesicles (tig. 104), and, on the 

 addition of water, exhibit an internal large 

 body resembling a nucleolus, as also several 

 granules. The minute details of this stage 

 of the process of formation are not accu- 

 rately known. As soon as the nuclei are 

 formed, cell-membranes are formed around 

 them, but not always in the same manner. 

 Sometimes the cell-wall is deposited directly 

 around the nucleus, so that it is but little 

 larger than the latter; at others the nucleus 

 becomes surrounded by a larger or smaller 

 quantity of protoplasm which becomes more 

 sohd. and around which the cell-membrane 

 is subsequently deposited. The latter oc- 

 currence has hitherto only been satisfacto- 

 rily observed in the case of the ovum, in 

 which the germinal vesicle, i. e. the nucleus 

 of the ovum-cell, which is formed first, 

 becomes surrounded by a quantity of yelk, 

 before the vitelline membrane is formed. 

 On the other hand, the formation of the 

 cell-wall directly around the nucleus has 

 been supposed to take place in all the other 

 localities mentioned above ; and to be espe- 

 cially shown by the occurrence of free 

 nuclei and larger cells, together with very 

 small cells closely surrounding the nuclei, 

 or separated from them by a slight interval 

 only. It is possible that, in this instance 

 also, the cell-membrane, even at its first for- 

 mation, may be separated from the nuclei 

 by a quantity of protoplasm too minute to 

 be detected. 



The free formation of cells has been ob- 

 served by Weissmann in insects during de- 

 velopment. 



The extracellular formation of cells is 

 unknown in plants. And probably, when 

 observers are agreed, it will be found that 

 all protoplasts and cells are deiiAed from 

 parent protoplasts by some form of segmen- 

 tation. 



The endogenous method, or the formation 

 of cells within others, is very common, and 

 may be readily observed in the tissues of 

 embryos. In the most ordinaiy foi-m of this 

 kind of cell-formation, an original or parent- 

 cell produces within it two secondary cells, 

 which from the first completely fill it. The 

 first phenomenon observed in the parent- 

 cell is the increase of the nucleus, which 

 acquires two nucleoli, becomes elongated 



and resolved into two nuclei. After this 

 the nuclei separate from each other, and a 

 partition is formed between the cells, di- 

 viding the parent cell into two perfectly 

 distinct spaces, each of which encloses a 

 nucleus and half of the contents (fig. 100). 

 The exact manner in which the increase of 

 the nucleus occurs is not certain ; but it ap- 

 pears that the nucleoli always become re- 

 solved into two by subdivision, and then 

 separate from each other. In the nuclei, 

 which at the same time become elongated, 

 the first trace of division is then usually a 

 median partition, which in favourable in- 

 stances appears to arise from the presence 

 of two secondary cells in close contact by 

 plane surfaces and entirely filling up the 

 parent nucleus. Very frequently nothing 

 is seen but first an elongated nucleus with 

 a partition and two nucleoli, and then two 

 hemispherical nuclei in contact by their 

 plane surfaces (fig. lO.o), no endogenous 



Fig. 105. 



Magnified 350 diameters. 



An elongated nucleus, and one containing two secon- 

 dary nuclei, from the ovum of an Ascaris dentata. 



nucleus-formation being perceptible ; in this 

 case, division of the nucleus has taken place, 

 the parent-nucleus containing two nucleoli 

 becoming finally resolved into two by a 

 deeper and deeper constriction. This mode 

 of cell -formation is often continuously re- 

 peated, frequently so long as the growth of 

 the organism continues. The parent-cells 

 then either continue their existence as such, 

 or they disappear sooner or later as histo- 

 logically distinct formations, and become 

 consolidated with the substance connecting 

 the ceUs. 



The endogenous cell-formation, which 

 agrees essentially with the formation of 

 cells in a free protoplasm, is well estabhshed 

 in the case of the young cartilages of all 

 animals ; it also probably occin-s in embry- 

 onic organs in general, in which, from the 

 period at which they consist of true cells, 

 the entire growth depends upon the multi- 

 plication of the existing cells without free 

 cell-formation. It also occurs in patholo- 

 gical products, as in cancer. 



In addition to this — the most common 



