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Dr. J. F. GemmilL The Anatomy oj 



The following is a short summary of the anatomy of the various 

 kinds of double monstrosity described : — 



Type 1. Union in head region — 



a. The twin trains united at the mesencephalon. 



b. The twin brains united at the medulla oblongata. 



Type 2. Union in pectoral region — 



a. The pectoral fins absent on adjacent sides. 



b. The pectoral fins present but united on adjacent sides. 



Type 3. Union behind the pectoral region — ■ 



a. The twin bodies united at a considerable distance in front of the vent. 



b. The twin bodies united close to the vent. 



Type 4. Union by the yolk-sac only. 



Type la shows the following characteristics : — 



The cerebral lobes and the thalamencephala are doubled. 



There are two infundibula, two hypophyses and two pairs of hypo- 

 aria. The optic lobes have a single cavity, but their basal parts show 

 marked evidence of duplicity. Cerebellum pons and medulla are 

 single, but there is a remarkable reappearance of duplicity in the cervical 

 part of the spinal cord. 



There are two pairs of 1st, 2nd, 3rd (and 4th) nerves, but only 

 single pairs of the 5th, 6th, 7th, 8th, and vagus nerves are present. 

 The cervical part of the spinal cord gives off in each segment a small 

 extra pair of ventral roots. 



There are two pairs of olfactory organs, all of which are normal. 

 There are also two pairs of eyes, the outer ones (right of right head 

 and left of left head) being normal. The inner or adjacent eyes (left 

 of right head and right of left head) lie close to one another, and are 

 more or less united. They have a common sclerotic and cornea, but 

 the retinae and choroids are separate. In some cases the lens is a 

 single composite structure ; in others it is doubled. Of eye muscles 

 the external recti are always, and the superior obliques are sometimes, 

 awanting. The other eye muscles are all present, and each eye has 

 its own optic nerve, choroidal fissure, choroidal gland and choroidal 

 artery. 



In front there are two sets of skeletal structures which converge 

 rapidly as one goes backwards. The adjacent trabecular, supraorbital, 

 and palatopterygoicl bars coalesce posteriorly, while the adjacent para- 

 chordals are united along their whole length. There are two pituitary 

 spaces. Only a vestige remains of the adjacent Meckelian cartilages. 

 The notochords are double in front and remain separate for about 

 twenty somites. They retain duplicity longer than any other 

 structure. Adjacent neural and costal arch cartilages unite, become 



