TERATOGENESIS. 615 



for its nutrition. The problem, however, is to explain the origin of the rudi- 

 mentary anlage which produces the parasite. In view of the facts concerning the 

 early stages of development — the facts concerning maturation, fertilization and 

 segmentation of the ovum, and the formation of the germ layers — there are two 

 possible and plausible modes of origin of the rudimentary anlage. (i) The 

 anlage of the parasite may be the result of the imperfect development of a 

 fertilized polar body. (2) The anlage of the parasite may be a special or an 

 isolated group of segmentation cells. 



1. It is generally agreed that the polar bodies are abortive or rudimentary 

 ova extruded during the processes of maturation of the female sex cell; and that 

 these rudimentary ova probably contain the same morphological constituents 

 as the mature ovum itself. It is also known that in a few of the lower forms 

 the polar bodies arc capable of being fertilized and undergoing a considerable 

 degree of development, and that in some of the higher forms (rabbit, dog) the 

 spermatozoa may exist for some time inside the zona pellucida in the vicinity 

 of the polar bodies. In view of these facts it does not seem impossible that in 

 a few exceptional cases in Mammals the polar bodies may become fertilized 

 and produce rudimentary anlagen capable of giving rise to parasites. Such 

 an anlage would naturally lie in close proximity to the larger normal anlage 

 and might readily become attached to or finally enclosed within it. As a 

 more remote possibility, the polar body might become enclosed between the 

 blastomeres and thus finally produce the parasitic anlage within the meso- 

 dermal tissue where it might become an inclusion in some internal organ, 

 such as the genital gland. A polar body has been found between the blasto- 

 meres (rabbit). (Bischoff, Assheton, Bonnet.) 



2. The view that the parasite may arise as the result of the development of a 

 special or an isolated group of segmentation cells has more advocates than the 

 view given in the preceding paragraph. One of the most interesting phases of 

 this theory is the view that tumors of the sexual glands, as well as those of other 

 regions, are the products of development of the germ cells as distinguished from 

 the somatic cells. In the skate it has been demonstrated that certain cells are 

 set apart at a very early period of development (during segmentation), which 

 are destined to give rise to the sex cells of the embryo, and which take no 

 part in its general development. Normally these special cells pursue a course 

 of development and differentiation which leads to the formation of the mature 

 sexual elements (ova or spermatozoa) of the individual, but do not participate 

 in its general development. From this one may conclude that the primitive 

 germ cell, the one set apart for the production of the mature sex cells, is, so 

 to speak, a sister to the embryo and is not a derivative of the embryo. It 

 seems not impossible that some aberrant members of this group of germ cells, 

 without undergoing the changes incident to maturation, might pursue a course 



