SECT. 2] AND WEIGHT 371 



Wt. of embryo + wt. of membranes 



Other sets of weight data have been reported by Lamson & Edmond, 

 by Murray and by Needham, for White Leghorn chicks, and by 

 LeBreton & Schaeffer for chicks of an unspecified race. These are 

 all placed in Table 3 of Appendix i, where it will be seen that the 

 general agreement between them is good. The values obtained by 

 Murray; Byerly^ and Schmalhausen are probably the most accurate, 

 for the conditions were very carefully controlled. Hanan's values are 

 lower than all the others. It is unfortunate that Schmalhausen does 

 not state what breed of hen was used in his experiments, though he 

 does mention that it was not genetically pure. Some early measure- 

 ments by Welcker & Brandt are not included in the table, for they 

 do not appear to be trustworthy. 



Other measurements which are useful are those of Edwards, who 

 has published a peaked curve showing the length of the primitive 

 streak during the first 50 hours of incubation. ^ Schmalhausen's work 

 on the growth of parts of the chick embryo will be dealt with later : 

 he was preceded by Falck, who measured and weighed various 

 organs but did not use enough material to make his figures valuable 

 to modern workers. 



Mammals. 



[a) Mouse. In 1923 LeBreton & Schaeffer published figures for the 

 embryonic growth of the mouse, but these were not very numerous. 

 The only other work on this subject is that of McDowell, Allen & 

 McDowell, probably the most accurate and satisfactory study of pre- 

 natal growth in any form that at present exists. Their figures are given 

 in Table 4 of Appendix i, and the curve obtained from them in 

 Fig. 22. This is drawn on arithlog paper, the ordinates in logarithmic 

 ruling giving the actual weights in gm., the abscissae in arithmetical 

 ruling giving the age and the number of the individuals. On each day 

 the range of the individual unclassified weights is shown by a vertical 

 line which is itself used as a base-line for the frequency distribution 

 of the classified individual weights. The number of cases in the 



^ See also Fischel and Leva. 



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