ONTOGENESIS 221 



i.e., the cephalic and caudal ends of the embryos 

 correspond. Sometimes, however, they rotate until 

 the cephalic ends are opposed and the caudal ends 

 conjoined, or the caudal ends opposed and the cephalic 

 ends conjoined. 



In rare cases the relation of one embryonal axis to 

 the other is lost and the attachment of one embryo 

 to the other without correspondence of the parts. 



The attachment of one individual to the other may 

 embrace the fundamental and vital organs heart, 

 brain, spinal cord, etc.; or may be through compara- 

 tively unimportant structures, the twins being conjoined 

 by a kind of slender pedicle, as in the Siamese twins. 



Some knowledge of embryology likewise enables one 

 to understand certain monstrous formations arising in 

 single individuals. Thus the bridging of the neural 

 canal to form the spinal canal in which the future spinal 

 cord is to lie, is one of the first features of mammalian 

 development. If this process fail anteriorly, the im- 

 perfect covering permits morbid changes, known as 

 craniorrachischisis, wi h meningocele or encephalomen- 

 ingocele, and terminating in anencephaly; when occur- 

 ring posteriorly, in my elo meningocele and spina bifida. 



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. 



