Symmetrical Double Monstrosities in the Trout. 131 



reduced in size, and finally disappear as one goes backwards. The two 

 outer series of cartilages are continued posteriorly into the single region 

 of the body. 



Head Kidney. — The glomerulus is sometimes double and sometimes 

 single ; when single it has two glomerular tufts, and is divided into 

 three chambers. Each of the outer chambers gives origin to a normal 

 Wolffian duct. The middle chamber is closed. When there are two 

 glomeruli, a normal Wolffian duct arises from the outer half of each 

 glomerulus, but the Wolffian ducts which should arise from the inner 

 or adjacent sides of the glomeruli are either entirely absent or are 

 represented only by short blind sacculated tubules. 



Alimentary Canal. — Two mouth openings lead into a single buccal 

 cavity. Pharynx, stomach, liver, and intestine are single, but there 

 are two air-bladder diverticula. 



Type lb. Union in Head Region, the brains being united at the medulla 



oblongata. 



The medulla and the fourth ventricle cavity bifurcate anteriorly 

 and lead to two separate sets of mid- and fore-brain cavities and 

 masses. Pons and cerebellum are double. There are two sets of 

 cranial nerves. The inner or adjacent elements of the 5th, 7th, and 

 8th pairs are reduced in size, while the corresponding vagi are 

 extremely rudimentary. The anterior part of the medulla is double ; 

 the posterior part is single and composite. The cervical part of the 

 spinal cord shows striking evidence of original duplicity, and has a set 

 of small extra roots coming off from its ventral aspect as in Type la. 



There are two pairs of olfactory organs and two pairs of eyes, all of 

 which are normal. The outer auditory organs (right of right head and 

 left of left head) are normal. In addition there is a small malformed 

 auditory organ placed in the angle between the two converging heads ; 

 it consists of united adjacent labyrinths and capsules, and has dis- 

 tributed to it on either side the small adjacent 8th nerves previously 

 mentioned. 



Cranial Skeleton. — In front, the cranial skeletal elements are in two 

 separate sets ; these converge posteriorly, their basal parts uniting at 

 the level of the medulla oblongata. There are thus two separate nasal 

 cartilages, two separate sets of trabecule cranii and two pituitary 

 spaces. The adjacent parachordal cartilages unite and form with the 

 outer ones a single plate which underlies the composite medulla 

 oblongata and covers the cranial parts of the two notochords. The 

 inner or adjacent palatopterygoids, supraorbitals, hyo-mandibulars 

 and periotic capsules are united and reduced in size. In the visceral 

 skeleton there are elements representing fused adjacent Meckelian and 

 hyoid bars, while the copular cartilage which succeeds the glossohyal is 



