TERATOGENESIS. 601 



ventricles. They possibly represent parasitic anlagen which have become en- 

 closed within the brain vesicles as the neural groove closed in dorsally. 



Certain foetal inclusions attached to the region formed by the branchial 

 arches are spoken of as cervical parasites. These are usually cystic tu- 

 mors, covered with skin and containing teeth, bone and parts of a head and 

 extremities. 



Closely associated with the cervical parasites is a group of tumors found 

 within the anterior mediastinum in the region of the thymus gland, and known 

 as thoracic parasites. It must be borne in mind, however, that some of the 

 tumors found in the cervical and thoracic regions are not true parasitic in- 

 clusions, but are dermoid cysts (resembling the parasites) derived from the 

 ectoderm. True parasitism implies origin from all three germ layers. From 

 a structural standpoint it is sometimes very difficult, even impossible, to distin- 

 guish between true parasitic inclusions and dermoid cysts that are derived from 

 ectoderm. 



Very rarely in the human subject some parasitic structure is attached to the 

 back. One case of a supernumerary penis in the lumbar region has been de- 

 scribed; another case is on record of an almost complete set of female genitalia 

 on the back of a male. Such malformations can be explained only by assum- 

 ing the partial development of another embryonic anlage. 



Sacral parasites are the most frequent of the true parastic growths. These 

 are cystic tumors which are attached to and hang from the sacrum or the 

 coccyx. The tumors are covered with skin which blends with the skin of the 

 autosite. In the existence of such elements as fat, bone, muscle, and nerves, 

 and the rudiments of intestines and extremities is found the evidence of their 

 fcetal origin. 



Foetal inclusions in the abdominal region are not frequent. One very rare 

 intraparietal (or subcutaneous) inclusion, in a child two and one-half years 

 old, proved to be a cystic tumor which contained a fairly well formed foetus 

 with defective head and extremities. Engastric (intraabdominal) parasites 

 are usually found in the region of the lesser peritoneal sac, at the root of the 

 transverse mesocolon. These tumors are usually enclosed within a sac of 

 mesenteric or peritoneal tissue. There may be well marked fcetal structures, 

 such as head, trunk, extremities, etc., or only traces of rudimentary organs. 

 The presence of an intraabdominal parasite does not necessarily cause the 

 death of the autosite immediately after birth; for one case in particular is on 

 record in which the autosite (a boy) lived to be fifteen -ears old with a parasite 

 that was capable of independent movement. 



Parasitic Structures in the Sexual Glands. The type of tumor referred to 

 here forms a group that is of especial interest owing to their relative frequency 

 of occurrence and to their peculiar mode of production. In connection with 



