THE CELL AND THE TISSUES. 23 



the ovum, when a single element penetrates the envelopes of the 

 egg and is received within the protoplasm of the female cell. The 

 head of the spermatozoon, which alone passes within the ovum, be- 

 comes transformed into the male pronucleus. Subsequently the 

 latter approaches the female pronucleus, the fusion of the two pro- 

 nuclei being followed by the appearance of a new nucleus of seg- 

 mentation, so called from the fact that within this body cleavage 

 of the ovum is first inaugurated. The segmentation nucleus con- 

 tains the chromatin contributed by the sexual elements of both 

 parents the fundamental fact in the consideration of heredity. 



The process of segmentation following the fertilization of the 

 ovum is essentially one of indirect cell-division, in which the 

 stages, although modified in certain details, are essentially the same 

 as those already described. The mammalian ovum undergoes a 

 total segmentation; although the resulting segments are, strictly 

 regarded, not quite equal in size, yet, as a matter of simplicity, they 

 may be regarded as such, and the division characterized as total 

 equal segmentation. 



The repeated cleavage of the segmentation-spheres into which 

 the ovum is divided soon produces a mass of innumerable cells con- 

 stituting the blastoderm ; the latter, by continued division and 

 further differentiation, subsequently gives place to a cell-area, in 

 which at first two layers, an outer and an inner, and later a third 

 middle stratum, of cells appear. These more or less imperfectly 

 defined tracts constitute the important primary blastodermic 

 layers, the ectoderm, mesoderm, and entoderm, from which are 

 derived all the tissues of the body. The reader must be referred to 

 the various text-books of embryology for a detailed account of the 

 complicated and often obscure processes of maturation, fertilization, 

 segmentation, and blastulation, of which only the most salient points 

 have been indicated above. 



THE TISSUES. 



Every tissue is composed of two parts, the cellular elements and 

 the intercellular substance. Upon the first of these depends the 

 vitality of the tissue, while its physical properties are determined by 

 the character of the second. The physical condition of the inter- 

 cellular substances includes a wide latitude, varying from that of a 

 fluid, as blood or lymph, through all degrees of density, until, by 

 the additional impregnation of calcareous matters, the well-known 

 hardness of bone or dentine is attained. 



The proportion between the cellular elements and the intercellular 

 substance of mesodermic tissues varies with age and development, 

 the intercellular substance in the early stages being scanty and very 



