DEVELOPMENT OF CELLS. 121 



cleus, as is generally stated, the cell-membrane is developed, and 

 thus the cell is completed. 



But, admitting the preceding account to be correct in particular 

 cases, another explanation of the process certainly applies in some, 

 and, it is believed, in most instances. It is thus expressed by Dr. 

 Burnett as being usual in animal tissues, at least : 



1. A primordial utricle appears in the clear plasma. 



2. It expands into a clearly seen vesicle. 



3. A partial condensation occurs of its liquid contents towards 

 the centre, giving rise to a new utricle, constituting the nucleus ; 

 the whole thus forming the complete nucleated cell. 1 



When we consider that the nucleus, the nucleolus, and the primary 

 utricle are all vesicles, or minute cells already formed, we may well 

 admit that either the utricle or the nucleus may be first formed, and 

 in time become a larger cell ; and it still admits of question whether, 

 in the last instance, the cell is not formed by mere enlargement of 

 the nucleus, instead of around it. But, doubtless, a cell, while being 

 developed, may also develop a nucleus within it, precisely as a 

 nucleus develops within itself a nucleolus. 



The most- satisfactory view of the subject appears to be the fol- 

 lowing ; it being understood that only those cells are now under 

 consideration which are formed in a clear plasma, and not those 

 formed from other cells. 



No sufficient reason appears why a primary utricle may not be, 

 in all cases, as easily formed in a clear plasma as a nucleus can; 

 and if this becomes a cell in one instance, and then forms a nucleus 

 within itself, it is hardly probable that a nucleus, on the other hand, 

 is first formed in other instances, and then forms a cell around itself. 

 It is far more probable that the first cells are formed in a clear 

 plasma without the aid of a nucleus, they being merely fully de- 

 veloped primary utricles; true/ree ce?/-formation always proceeding 

 in this way. The second generation of cells may be formed from 

 the nuclei in the first, or not ; but, if so, it is endogenous formation, 

 and not the form of development here under consideration. If all 

 the cells of any given kind are to be formed by free cell-develop- 

 ment, however (as the chyle-corpuscles), then we see no advantage 

 at all in their containing nuclei. The fact that any cell contains a 

 nucleus evinces a power of producing another cell by endogenous 



1 Prize Essay, Transactions of the American Medical Association, vol. vi. p. 861. 



