608 TEXT-BOOK OF EMBRYOLOGY. 



not the result of fission but of bifurcation which accompanies the development 

 of the head end of the embryo along divergent axes. The difference between 

 fission and bifurcation is that the former is the passive result of active 

 mechanical forces, while the latter is a part of active formative processes. 



Experiments on eggs of lower animals point to the conclusion that each of 

 the two blastomeres resulting from the first cleavage contains the material 

 necessary to produce an entire body. In order to cause a blastomere to pro- 

 duce a whole body, however, it is necessary to subject it to unnatural conditions. 

 For example, if one of the two primary blastomeres of the frog is killed and the 

 other left to grow in its natural position it will produce a half-embryo; but if the 

 remaining blastomere is inverted it will produce a whole embryo (Morgan). 

 On the other hand, in view of certain experiments on the eggs of Amphioxus, 

 it has been asserted that duplicity is associated with double gastrulation; 

 when these eggs were shaken during the first cleavage, some developed into 

 double gastrulae (Wilson, Hertwig). For a further discussion of these causes, 

 see page 612. 



ASYMMETRICAL DUPLICITY. 



In this type of malformation the two anlagen from which the monster is 

 derived are primarily dissimilar and unequal. One anlage usually remains in 

 a rudimentary condition, bears little or no resemblance to a foetus, and 

 becomes a parasite upon or within the body derived from the other anlage 

 (parasite, foetal inclusion, foetus in fcetu). Sometimes, however, the dependent 

 embryo may develop quite complete parts, such as extremities, but always 

 remains attached to the stronger embryo, from which it derives its nourishment 

 (implantation). Parasitic inclusions and implantations may be attached to 

 the autosite in the region of the head, neck, thorax, abdomen, etc. 



Parasitic duplicities in the head region may take the form of masses pro- 

 truding from the orbital region prosopopagus parasiticus or much more com- 

 monly from the mouth epignathus, sphenopagus. In the latter case the tumor 

 is enveloped by skin containing hair follicles and sweat glands, and usually 

 consists of cysts and intervening embryonic tissue. It sometimes contains teeth, 

 cartilage, bone, fat and nerve tissue, even traces of an intestinal canal and 

 of liver tissue. One epignathus has been described as having an imperfect 

 set of female genitalia which lay between two rudimentary lower extremities. 



Occasionally irregular tumors are found in the region of the pituitary body 

 (encranius), which contain rudiments of various tissues and organs. In such 

 cases the parasitic anlage has possibly been included during the imagination 

 which forms the oral part of the pituitary body. Tumors consisting of various 

 tissues, such as lymphatic, adipose, muscle, etc., are also found in the. brain 



