602 TEXT-BOOK OF EMBRYOLOGY. 



the ovary dermoid cysts and other solid masses occur. The cysts consist of a 

 sac enclosing hair and adipose tissue; not infrequently teeth are also present, as 

 well as sebaceous and sweat glands. Sometimes there are also bone, cartilage, 

 muscle, and nerve tissue and traces of the digestive and respiratory systems and 

 of thyreoid gland; more rarely traces of mammary glands, finger nails and 

 retinal pigment are present. In the rarer solid tumors that develop in rela- 

 tion to the ovary all three germ layers are represented, but their derivatives 

 are more rudimentary and not so regularly arranged as in the cysts. 



Parasitic growths in the testis are much less frequent than in the ovary. 

 The cysts are rarer than the solid masses. These are probably homologous 

 with the parasites of the ovary. 



ORIGIN OF ASYMMETRICAL (PARASITIC) DUPLICITY. 



Parasitic duplicity implies primary inequality of the embryonic anlagen; in 

 other words, the anlage of the parasite is inferior, so to speak, to the anlage of 

 the host. During development the inequality or asymmetry persists or be- 

 comes more conspicuous until the parasite is more or less enclosed within the 

 autosite. As the autosite develops in a manner at least simulating the normal, 

 the parasite remains in a more or less rudimentary condition, with perhaps only 

 a few tissues which show any differentiation. In some cases the parasite 

 becomes enclosed partially or completely within the autosite (epignathus) , in 

 other cases the parasitic growth apparently occurs primarily within the autosite 

 (ovarian cysts). 



The manner in which the rudimentary anlage becomes surrounded by the 

 more nearly perfect anlage is, of course, not known through direct observation. 

 But it seems reasonable to assume that such enveloping occurs in connection 

 with or as a part of the normal processes of folding by which the external form 

 of the body is established. This folding occurs at the cephalic and caudal poles 

 of the embryonic disk and also along its entire length. In addition there is 

 also the folding in of the neural groove along the dorsum of the embryo, and 

 the invagination of the branchial grooves. One can readily imagine the para- 

 sitic anlage as attached to some one of the areas that are folded in, so that 

 it is carried wholly or partially into the interior of the stronger embryonic 

 anlage and becomes surrounded by the tissues of the autosite to produce a 

 true foetal inclusion. 



There seems to be little doubt as to the existence of a second, more or less 

 rudimentary anlage which becomes the parasite; in other words, there is almost 

 certainly a duplicity to begin with, although it may be an asymmetrical one. 

 It is also plausible to assume that for a time the weaker anlage develops in- 

 dependently of the stronger, but that later it is dependent upon the stronger 



