THE CELL DOCTRINE. . 101 



fcEtus reaching in a short space of time the dimen- 

 sion of .030 millimeter (.0118 of an inch). 



4th. Germination. — This is very frequent in vege- 

 tables, but in animals only one exam-pie is known, 

 viz., at a period previous to the fecundation of the 

 ovum. Before segmentation takes place the vitellus 

 is observed to retract. The hyaline substance pushes 

 out a prolongation, which becomes round, separates, 

 and constitutes, an independent anatomical element 

 exterior to the vitellus, and bearing no part in the 

 future development of the ovum. 



The following account of certain special elements 

 illustrates and further explains the views of M. 

 Robin. Red blood-globules (hematics), dianieter, .007 

 millimeter (^-'n^ of an inch); thickness .002 milli- 

 meter (t2^52 of an inch). Blood-globules are elastic, 

 — a property enabling them to elongate, and pass 

 through capillaries which have a calibre less than 

 the diameter of the blood-globule. They are homo- 

 geneous throughout — i. e., have no cell-wall. Blood- 

 globules are formed by genesi^ in tHe blood ■plasma. In 

 the foetus they make their appearance before the 

 white blood-globules (leucocytes). In man there are 

 two kinds of red blood-globules, viz. : first, embry- 

 onic ; second, normal. The embryonic blood-globules 

 are double the size of the normal ones. They have 

 a slightly granular nucleus, situated nearly in the 

 centre, which is insoluble in acetic acid. The nor- 

 mal blood-globules are not a transformation of the 

 embryonic. They appear by genesis in the midst of 

 the blastema of the blood. After the fourth month, 

 the embryonic globules cease to form, and as the 



