SECT. 2] 



AND WEIGHT 



375 



Watson in their earlier paper, for, in assuming that embryonic growth 

 in the rat followed a quite similar course to that taking place in 

 man and the rabbit, they did not allow for the long time taken for 

 the rat egg to pass through the oviduct after fertilisation. Thus they 

 arrived at the result that the rat embryo of 15 days should probably 

 weigh 2-6 10 gm., whereas by direct measurement Stotsenberg found 

 that it only weighs o- 1 68 gm. Their calculated figures are consequently 

 not included in Appendix i. 



(c) Guinea-pig. The most usually quoted work on the embryonic 

 growth of this animal used to 

 be that of Read, who used 

 a very indirect method of 

 measuring it. He weighed the 

 pregnant female every day be- 

 tween insemination and birth 

 and then each foetus with its 

 membranes and fluids, from 

 which data, assuming that 

 growth had taken place regu- 

 larly, the weight of one embryo 

 could be calculated. He con- 

 cluded that the guinea-pig 

 passes through two growth- 

 cycles during its intra-uterine 

 life. But no satisfactory conclu- 

 sion can really be drawn from 

 such figures, subject as they are 

 to all kinds of complicating 

 factors, and, like the earlier 

 ones of Minot on the guinea- 

 pig, obtained in the same in- 

 direct way, they are better 

 discarded. It is needless to point out that differences in the weight 

 of mother + embryo due to defaecation, filling of caecum, etc., may 

 amount to grams, while the weight of the embryo is still only 

 milligrams. In the absence of any other figures, they had their 

 importance, but in 1920 Draper made a complete study of the 

 embryonic growth of the guinea-pig. Together with the few frag- 

 mentary (but direct) figures of Hensen, and the careful work of 



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40 



50 



60 



70 



Fig. 25. 



