132 MANUAL OF HISTOLOGY. 



ire still destitute of haemoglobin, which gives them their characteristic 

 peculiarities at a later period of their existence. Their size is also very 

 various, exceeding frequently that of the cells of fully developed blood. 

 Their average diameter in the embryo of fowl is, according to my obser- 

 vations, about 0*0128 mm. 



Little by little the cell becomes clearer, and the characteristic yellow 

 tinging with haemoglobulin commences, this substance being deve- 

 loped by the body, of the former. Such coloured nucleated cells range 

 in their diameter in the human being and in mammals, from 0'0056 to 

 0-0160 mm. (Paget, Koiliker). 



Whilst this transformation of embryonic cells into blood-corpuscles is 

 proceeding with the further development of the circulatory system, the 

 blood must of course contain, at the same time, both kinds of cells, 

 the coloured as well as the more advanced, besides immature colourless 

 ones. 



During the earlier periods of foetal life, however, rapid multiplica- 

 tion of the red cells by means of division takes place, as was first observed 

 by Remak. This may easily be followed in the embryonic chick. 



The process begins here with division of the nucleolus, then follows 

 that of the nucleus, which generally splits into two portions, and only 

 very, seldom, according to Remak, into three or four. Sometimes such 

 a nucleus will divide anew, but it requires a very close search to discover 

 in the chick cells engaged in more than the usual binary division. Finally, 

 the contractile body of the cell is seen to undergo constriction in the 

 middle until the two portions part company. The extreme delicacy of 

 these blood-cells frequently gives rise to artificial appearances, for 

 instance, of cells which are furrowed in the .middle, and only contain a 

 nucleus in one-half, or others whose two portions containing nuclei are 

 held together by a long, thin, connecting thread. In the foetal chick it 

 is just in these periods of formative life in which the liveliest increase of 

 blood takes place, that this process of division (which may, as it appears, 

 pass over very rapidly) can be best observed. Later on, at a more 

 advanced stage of development, it ceases altogether, according to Remaks 

 and our own observations. 



We owe much to Koiliker for his profound in- 

 vestigations in regard to the blood of the mam- 

 malia. Of the correctness of his views I con- 

 vinced myself years ago in embryonic deer (fig. 

 125), as well as later on in rabbits and the human 

 embryo. Here also the same process of division 

 may be recognised. According to Remak, multi- 

 nuclear cells occur also frequently. To me the 

 nuclei always appear granular. Moreover, the act 

 of division is liable to temporary variations, it ap- 

 pears. Thus in rabbit embryos of 9 mm. long I 

 have only remarked a very small number of cells 

 fig. 125. -Biood-ceiis from the engaged in division, while the process could be 



early embryo of the deer. The often recognised in much lamer Ones. The fur- 

 more spherical cells at a a a; ,, , ,. p ,, 



multiplication of the same from ther destiny of these in general st ill larger cells 

 (though they may vary considerably as to dimen- 

 sions), consists in this, that they take on more and more the spherical form, 

 lose their inequality as to size, and assume the typical shape ; the nucleus 

 disappears at the same time in mammals. Even at a very early period 



