THE TISSUES 



537 



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gical elements of every tissue are modifications or products 

 of such cells ; (2) that every tissue was once a mass of such 

 cells more or less closely packed together ; and (3) that the 

 whole embryonic body was at one time nothing but an 

 aggregation of such cells. 



3. The Body starts as a Single Cell, the Ovum, 

 which then divides into primitive cells.— The body of a 

 man or of any of the higlier animals commences as an ovum 

 or egg. This (Fig. 173) is a minute transparent spheroidal 

 sac 200/i, {j^Q of an inch) in diameter in man, which con- 

 tains a similarlj- spheroidal mass 

 of protoplasm, in which a single 

 large nucleus is imbedded. 



The first step towards the 

 production of all the complex 

 organisation of a mammal out 

 of this simple body is the di- 

 vision of the nucleus into two 

 new nuclei which recede from 

 one another, while at the same 

 time the protoplasmic body be- 

 comes separated, by a narrow 

 cleft which runs between the 

 two nuclei, into two masses, or 

 blastomeres (Fig. 174, a), one for each nucleus. By the re- 

 petition of the px-ocess the two blastomeres give rise to 

 four, the four to eight, the eight to sixteen, and so on, 

 until the embryo is an aggregate of numerous small 

 blastomeres, or nucleated cells. These grow at the 

 expense of the nutriment supplied from without, 

 and continue to multiply by division according to 

 the tendencies inherent in each until, long before 

 any definite tissue has made its appearance, they 

 build themselves up into a kind of sketch model of the 

 developing animal, in which model many of, if not all 

 the future organs, are represented by mere aggregates of 

 undifferentiated cells. 



Fig. 173.— Diaukam of the 

 Ovum. 



a, granular protoplasm ; 

 6, nucleus, called "germinal 

 vesicle " ; c, nucleolus, called 

 " genninal spot.' 



