CHAP. I.] DEVELOPMENT OF THE TISSUES FROM CELLS. 49 



made out. The germinal vesicle and spot become the seat of a 

 series of changes, which give rise to the development of new cells, 

 for t^he formation of the embryo. 



At this period the embryo consists of an aggregate of cells, and 

 its further growth takes place by the development of new ones. 

 This may be accomplished in two ways : first, by the development 

 of new cells within the old, through the subdivision of the nucleus 

 into two or more segments, and the formation of a cell around 

 each, which then becomes the nucleus of a new cell, and may in its 

 turn be the parent of other nuclei ; and, secondly, by the formation 

 of a granular deposit between the cells, in which the development 

 of the new cells takes place. The granules cohere to each other in 

 separate groups here and there, to form nuclei, and around each of 

 these a delicate membrane is formed, which is the cell-membrane. 

 The nuclei have been named cytoblasts, because they appear to form 

 the cells (KVTOS, cell ; /SXacrreo), to produce) ; and the granular de- 

 posit in which these changes take place is called the cytoblastema. 



In every part of the embryo the formation of nuclei and of cells 

 goes on in one or both of the ways above-mentioned; and, by and 

 by, ulterior changes take place, for the production of the ele- 

 mentary parts of the tissues. The precise share which the cells 

 take in this process cannot be made intelligible in the present stage 

 of our inquiry, even if observers were agreed in their accounts of 

 the phenomena. It must suffice for us now to explain, as far as 

 we are able, the general changes that occur, and the probable 

 office which each part of the cell performs in them. 



The changes which the cells undergo in the formation of the 

 tissues, may be described under two heads : first, those affecting 

 the cell-membrane ; and, secondly, those in which the nucleus is 

 concerned. In those tissues, whose ultimate elements are fibrous, 

 that is, consisting of real or apparent fibres, as areolar and fibrous 

 tissues, the cell-membranes become elongated, and so folded or 

 divided as to give the appearance of a subdivision into minute 

 threads or fibres. In the tissues, which are composed of tubes of 

 homogeneous membrane containing a peculiar substance within 

 them, as muscle and nerve, the cells are joined end to end, and, the 

 partitions at each extremity being removed, their cavities com- 

 municate, so that they together form a tube, or sheath, in which 

 the deposit of the proper muscular or nervous substance takes 

 place. The smallest or capillary blood-vessels also are formed by 

 the coalescence of the walls of the cells, not at one or two, but 

 at several points, owing to their elongation, here and there, into 



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