SECT. 2] 



AND WEIGHT 



373 



Foetus of albino rab 



will be discussed later, in Section 2-4 (p. 393). The lay-out so 

 made reveals several interesting features; it appears, for example, 

 that there are always individuals on a given day which are equal 

 in weight to the mode of the day before. McDowell, Allen & 

 McDowell consider that this is evidence of a possible delay of as 

 much as 24 hours between copulation and fertilisation, but, whether 

 this is so or not, it certainly equates with exactly similar variations 

 found in the chick both in the early stages (primitive streak) and in 

 the later ones of organ-growth. Further, the modes and means are 

 generally close together, though less so at the beginning of develop- 

 ment than at the end, and the latter do not approximate to a straight 

 line. A glance at the graph also shows that the highest individual 

 weights on each day tend to form a curve parallel with that of the 

 means throughout development. 



(b) Rat. Donaldson's comprehensive monograph of 191 5 includes 

 a discussion of the growth of the rat embryo, but much less work 

 has been done on this animal 

 than upon man, for in the latter 5 

 case the ad hoc labours of ob- 

 stetricians have often provided 

 much valuable material for the 

 biologist. However, Stotsen- 

 berg's work gave a good account 

 of the matter, and his figures 

 are reproduced in Table 5 of 

 Appendix i, and in Fig. 23. 

 They begin from the 13th day 

 after insemination, before which 

 weighing is difficult, and they 

 continue until birth, which takes 

 place at the 22nd day. This pre- 

 natal period would appear to 

 be one complete growth-cycle, if we may judge from the work of 

 Donaldson, Dunn & Watson on the post-natal growth of the rat. 



Huber has studied the growth of the rat embryo in its earliest 

 stages prior to fixation to the uterine wall. He states that the egg- 

 cell of the rat approaches the uterine end of the oviduct while in 

 the two-cell stage, segmentation being slow and proceeding as the 

 transit takes place. Fig. 24, reproduced from his monograph, is a 



16 17 18 

 Fig. 23. 



