GENERAL HISTORY OF DEVELOPMENT. 471 



fortnight is occupied in preliminary processes, no trace of the 

 embryo appearing until the twelfth or thirteenth day. From 

 the end of the second to the end of the fourth week, the embryo 

 is acquiring definite form, and the various organs and systems 

 are being established. From the fourth to the sixth or seventh 

 week there is a gradual change from the embryonic to the foetal 

 form ; the head becoming uplifted, the nose, ears, and lips esta- 

 blished, the limbs divided by joints, and the fingers and toes 

 formed. By the end of the second month, the general form is as 

 shown in Fig. 212, and from this time onwards the further 

 changes consist chiefly in increase of size, and in proportionately 

 greater development of the limbs. 



The changes that occur in the shape and size of the embryo 

 up to the end of the second month are well shown in the series 

 of outlines given in Figs. 176 to 178, 189 to 195, 199 to 203, 

 205, 211, and 212. These figures, which are borrowed from 

 Professor His, are in each case five times the linear dimensions 

 of the embryos themselves. 



1. The First Week. 



The fertilisation of the human ovum has not been studied. 

 A single observation, by Nagel, of a ripe ovarian ovum, removed 

 by operation and examined in a fresh condition, showed that 

 two polar bodies were present, lying on the surface of the ovum 

 within the zona pellucida. 



There is no reason for supposing that fertilisation is effected 

 in other than the normal manner; and it is probable that it 

 takes place at or about the time the ovum leaves the ovary and 

 enters the oviduct. 



The segmentation of the human ovum has not been seen. It is 

 highly probable, from analogy of other Mammals, that it occurs 

 during the passage of the ovum along the Fallopian tube towards 

 the uterus. 



The ovum of the dog, which is slightly smaller than the 

 human ovum, travels quickly along the first part of the oviduct 

 but stays some days in the distal or lower part, where it under- 

 goes segmentation, entering the uterus eight or ten days after 

 leaving the ovary. Bischoff and others believe, though there is 

 no direct evidence on the point, that the human ovum agrees in 

 this respect fairly closely with that of the dog ; undergoing 



