20 STRUCTURE AND GROWTH 



tins f;ict explains the common appearance of two or four cells 

 tying together in a group, being separated from one another 

 by thin Avails, whilst between such groups and the neighbour- 

 in"- cells we see much more intercellular substance. 



The cells at first appear finely granulated, and not so trans- 

 parent as in the more fully developed condition. The thick- 

 ening of the cell-membrane takes place simultaneously with 

 its expansion. One of the cells in pi. Ill, fig. 1, exhibits two 

 nuclei, one of which, like those of all the other cells, has but ; 

 one nucleolus, the other having two. It may be conjectured, ' 

 that this second nucleus is destined to the formation of a young 

 cell within the larger one. 



In the intercellular substance at e in the same figure (pi. Ill, 

 fig. 1,) may be seen a small corpuscle, surrounded by a granu- i 

 lous and indistinctlv circumscribed mass, the rest of the inter- \ 

 cellular substance being smooth and homogeneous. This is, 

 perhaps, a nucleus in the act of formation, the nucleolus of 

 which is already developed ; and when the granulous mass sur- 1 

 rounding that structure has obtained a defined external boun- j 

 darv, it will form a nucleus. If such be the case, we have \ 

 here an instance of accordance of the development of the germ 

 itself with the formation of the nucleus of vegetable- cells ob- ' 

 served bv Schleiden. 



On examining the cartilage of the branchial arches of the 

 tadpole in the more completely developed state, (pi. I, fig. 8,) 

 we find the cells generally lying in groups, so that two, three, 

 or four lie close together, separated from other groups by 

 thicker partition walls. The special walls of the individual 

 cells are less distinct, but at several spots where three or more 

 cells are in contact, for example, at a, the separation of the 

 walls may yet be seen, and a trace of intercellular substance is 

 also present ; the latter, however, is almost homogeneous with the 

 cell-walls. It may also be observed that the cell-walls are thicker, 

 in these situations than they are represented in pi. Ill, fig. 1. 

 Some parallel lines may be seen at various spots in these con- 

 densed cell- walls, and the thickening might, in such instances, 

 be supposed to be really produced by a stratified deposition of 

 the substance upon the internal surface of the cell-wall. But 

 at the same time it must be remembered, that every partition- 

 wall between two cells must consist of two layers, each of which 



