616 TEXT-BOOK OF EMBRYOLOGY. 



of development of their own accord and give rise to a rudimentary twin — the 

 foetal inclusion or parasite. In this case one must regard the germ cells as pos- 

 sessing an inherent potentiality which may institute formative processes; but the 

 actual cause of the spontaneous development is unexplained. (Born, Wilson, 

 Morgan, Driesch, Schultze). 



Another possible source of parasitic growths is suggested by experiments in 

 which some of the cells during segmentation have been separated from the 

 general mass. The artificially segregated cells may develop into perfect em- 

 bryos smaller than the normal, or into partial embryos. Further experiments 

 along the same line on the frog justify the assumption that the segregated cells 

 or masses may become enclosed within the developing larger embryo and there 

 undergo further growth and differentiation and give rise to inclusions or 

 parasites. (Roux.) 



As a matter of fact there seems to be no good reason for considering any one 

 of the above views as expressing the only possibility as to the source of unequal 

 duplicities or parasitic growths. There is nothing to show that all three 

 methods may not contribute to the various kinds of duplicities including 

 certain teratomata of the sexual glands. 



MALFORMATIONS INVOLVING ONE INDIVIDUAL. 



Description, Origin. 



While the more limited malformations and anomalies affecting individual 

 organs are discussed in the chapters dealing with those organs, it seems best 

 to consider here some of the gross malformations in a single individual, 

 especially those which affect the external form of the body. 



Defects in the Region of the Neural Tube. 



The term cranioschisis has been given to a group of malformations, or 

 defects, in the roof of the skull and in the brain. Depending upon the degree 

 of defect, the group is divided into two classes— acrania and hemicrania— 

 which include conditions from a complete absence of the roof of the skull to 

 partial arrest of development. Associated with these conditions are varied 

 defects and malpositions of parts or of the whole of the brain. 



In extensive acrania the entire roof of the skull is lacking, and the brain 

 and its membranes are reduced to small masses of tissue lying upon the floor 

 of the skull. The defect may also extend to the cervical vertebra— cranio- 

 rachischisis. These vertebrae remain open dorsally and are bent inward 

 (lordosis). The ears are set upon the shoulders and the neck seems to be 

 lacking. 



