30 ABNORMAL CONDITIONS OF THE AXIS. 



Subclass IV. Heterologous union. 



A. By Impaction. 

 Syn. Heterodidymus, Gurlt. 



A second embryo may become wedged in between the visceral 

 arches or laminae of another before they unite. In such cases 

 the impacted foetus usually exhibits great deficiency,, and is called 

 a parasite. Most frequently it has neither a heart nor a head ; 

 sometimes it becomes converted into a more or less amorphous 

 tumour, with one or two organs only developed in a normal 

 manner. The supporting foetus is called the autosite. 



a. Between the lateral halves of the sternum. 

 Syn. Thoracopagus Heterodelphus, Geoff. 



The upper part of the parasite is wedged in between the thoracic 

 laminae of the autosite., and lodged in the middle of a fissured ster- 

 num. The parasite is usually very defective j the axis may be en- 

 tirely wanting, the limbs only being developed. More frequently 

 there is a rudimentary abdominal cavity in the parasite, commu- 

 nicating with that of the autosite above the umbilicus, as in ordi- 

 nary thoracopagus. The abdominal cavity of the parasite is some- 

 times distinct from that of the autosite, and has a separate um- 

 bilicus. A perfect series of transitional forms exist between this 

 condition and ordinary thoracopagus ; hence it has been called 

 parasitic, or heterodelphous thoracopagus. 



123. A female foetal Pig with a headless acardiac parasite united 



to the thorax and abdomen. The cervical region of the 

 parasite is attached to the sternum of the autosite. The 

 body-cavities of the two are continuous. The parasite 

 has a single large kidney, a urinary bladder, and a pair 

 of hypogastric arteries. There is also a small piece of 

 intestine connected with that of the autosite by a long, 

 narrow gut. The abdominal cavities have been laid open 

 to show their continuity. Hunterian. 



124. A foetal Kitten with a similar parasite. Hunterian. 



125. A foetal Kitten with a parasite attached to the sternal region. 



The body-cavity of the parasite is very small and distinct 

 from that of the autosite. There are two distinct cords 



