8 HUMAN EMBKYOLOGY. 



tractile substance which forms the "physical basis of life." It consists of C., H., N., 0., 

 and S., combined together in different ways and in differing proportions to form 

 various modifications of protoplasm which possess definite physical and chemical 

 characteristics, and which receive, therefore, different names. 



The cell body consists of a kind of protoplasm called cytoplasm, separable 

 into two parts; the spongioplasm or cyto-reticulum, which forms a network or 

 spongework ; and a more fluid part, the hyaloplasm or cytolymph, which occupies 

 the interstices of the reticulum. 



The nucleus lies in the cytoplasm. It consists of a form of protoplasm, called 

 karyoplasm, which is separable into a more fibrillar part, the karyo-reticulum, and 

 a more fluid part, the karyo-lymph or nuclear juice. The reticulum also consists 

 of two parts, the achromatic or non-stainable part formed of a substance called 

 linin, and a part called chromatin, which is readily stainable. 



Chroma tin varies in appearance at various stages of the cell life. During the 

 resting periods, which intervene between the periods of cell division, it is broken 

 up into small particles which either are embedded in or are in close association 

 with the linin network. 



When cell division commences the chromatin particles are, in many cases, 

 aggregated to form a thread-like strand, which ultimately breaks up into a 

 number of segments called chromosomes. The chromosomes are probably of definite 

 number in the body cells of any given species of animal. In the human 

 subject the typical number is probably 24. 



According to Winiwarter's recent observations the number of chromosomes in each oocyte I 

 (see p. 12) is 48, and in each spermatocyte I (see p. 12) it is 47. Each mature ovum (see p. 13), 

 therefore, has 24 chromosomes, but some spermatids (see p. 17) have 24 and others 23. If a 

 spermatozoon (see p. 17) with 24 chromosomes unites with a mature ovum a female results, but if 

 a spermatozoon with 23 chromosomes unites with a mature ovum a male results. 



During the resting period the nucleus is bounded by a distinct nuclear mem- 

 brane, which is continuous on the one hand with the karyo-reticulum, and on the 

 other with the cyto-reticulum. 



The nucleolus is a spherical vesicle which lies in the karyo-lymph during 

 the resting periods of the cell. It disappears entirely during the periods of 

 division. The protoplasm of which it is formed is called pyrenin. In some cases 

 several nucleoli are present. 



The nodes of the karyo-reticulum are sometimes called false nucleoli. 



The centrosome is a clear spherical area of the cytoplasm which lies usually 

 in the neighbourhood of the nucleus. Around it the granules of the cytoplasm 

 are arranged in radial lines, and in its interior lie one or two minute, deeply 

 staining bodies, the centrioles. The centrosome appears to play a very important 

 part in cell multiplication ; and, in the more ordinary form of cell division, it 

 divides before the division of the cell takes place, but in certain cases it disappears 

 before the cell divides. 



The mitochondria are minute particles. They are demonstrable in the 

 majority of cells during life ; or by means of certain stains, after special methods 

 of fixation and preservation have been used. They are believed to play an 

 important part in the economy and life-history of the cells, and they form a very 

 definite part of the structure of the spermatozoon or male gamete. 



THE LIFE-HISTOKY AND CAPABILITIES OF ANIMAL CELLS. 



Every animal cell is formed by the division of a pre-existing cell called the 

 mother cell. The mother cell divides into two equal parts the daughter cells, each 

 of which, under ordinary conditions, possesses all the capabilities of its mother. 



Reproduction of Cells. Ordinary tissue cells increase in number by the 

 division of the pre-existing cells into equal parts, and each part possesses similar 

 capabilities. Every new cell has a definite life -history; it grows, performs its 

 proper function, and ceases to exist, either by dividing into two daughter cells, 

 or by dying and breaking up into fragments which disappear. 



Whilst the multiplication rate exceeds the death-rate in any given tissue or 

 organ, that tissue or organ grows. When the multiplication rate and the death- 



