240 BIOLOGY: GENERAL AND MEDICAL 



Partial failure of the anterior concrescenses by which 

 the face is formed explains the occurrence of hare-lip 

 and cleft palate and similar incomplete fusion of the 

 splanchnopleures and of the folds which form the geni- 

 talia, the occurrence of extrophy of the bladder, epis- 

 padias and hypospadia. 



The rare cases of reversed viscera, in which the heart 

 is in the right side, the liver in the left, and the spleen 

 in the right situs inversus viscerum are so perfect in 

 detail of structure, apart from the reversed condition, 

 that they cannot be enumerated among the monstrosi- 

 ties, but must be looked upon as referable to occasional 

 differences in the right or left impulse of growth in the 

 respective eggs. This coincides with Conklin's observa- 

 tions upon snails' eggs. 



Sex Determination. A peculiarity of development to 

 which no reference has thus far been made is that of the 

 sex of the individual, and as it is of importance for the 

 safety of the species that the number of individuals of 

 each sex be properly proportioned, it becomes a matter 

 of considerable interest to discover, if possible, how the 

 sex of each individual is determined. 



The question has received attention from early times, 

 and the number of theoretical explanations that have been 

 suggested corresponds with the obscurity and difficulty 

 of the problem. "Blumenbach, in his fascinating brochure, 

 'Ueber den Bildungstrieb,' points out that Drelincourt 

 brought together two hundred and sixty-two groundless 

 hypotheses of sex, that had been proposed, and Blumen- 

 bach remarks, quaintly, that nothing is more certain than 

 that Drelincourt's own theory made the two hundred 

 and sixty-third." 



At the present time, through cytological studies, we are 

 no doubt much nearer to the truth than at any time in 

 the past, though we are still unable to formulate a theory 

 of sex determination that is generally applicable. 



An analysis of the writings upon the subject may be 

 briefly synoptized as follows: 



