34 TRIPLE MONSTROSITY— STRUCTURE 



The distortion affects chiefly the ventral aspect of the tail and the tail fin. The ventral 

 edge membrane (including the anal fin) may be described as having been split widely apart into 

 two halves. These unite with a moiety from the corresponding structures of the defective 

 embryos, to form two composite ventral edge membranes. (Compare with ventral union in double 

 monstrosity, pp. 22-23.) A transverse section near the commencement of the tail (PI. VII. fig. 32) 

 shows five angles, viz. three referable to the dorsal edge membranes of the three embryos, and two 

 to the composite structures which have just been named. Nearer the tail (PL VII. figs. 33-34) the 

 number becomes reduced to four through fusion of the dorsal membranes of the two defective 

 embryos. 



The alimentary canal and the Wolffian structures are referred to, after the description 

 of " B." 



Defective Embryo "A." The cerebral lobes, the pineal body, and the 3rd ventricle region are 

 entirely absent. The optic lobes are small and distorted, while the cerebellum, the 4th ventricle 

 and the medulla are present but reduced in size. The spinal cord consists of irregular nervous 

 tissue without a central canal, and is not separated by a membranous capside from the surrounding 

 structures. It joins the spinal cord of the other defective embryo a short distance in front of 

 the vent. The united part becomes rapidly smaller, and disappears without passing back into 

 the tail. 



There is no trace of olfactory organs or of eyes, but two small otocysts are found lying against 

 one another below the medulla. The left otocyst has a main chamber without distinction into 

 saccule and utricle. It gives rise to a single large semicircular canal with an ampulla and crista 

 acoustica, besides two other small pockets destitute of sensory cells. The right otocyst shows traces 

 of division into saccule and utricle. The saccule has a sensory patch, while the utricle has one 

 well-developed semicircular canal besides a small blind evagination. 



A single small auditory nerve is seen passing down from the medulla between the adjacent 

 surfaces of the pair of otocysts. 



The notochord is absent except between the otocysts and for a short stretch below the 

 medulla. 



There are three or four irregular gill arches separated by slits and carrying gill tufts. Pectoral 

 fins are present, but reduced in size. There is a small single pelvic fin on the right side. 



The embryo is acardiac, and its tissues are supplied with blood from the yolk-sac vessels of the 

 principal embryo. 



Defective Embryo " E." The structures found in " B " are practically the same as in " A." The 

 spinal cord, however, is even more irregular, and the left notochord is entirely absent. A single pelvic 

 fin is seen on the left side of " B," and is in all probability a composite structure formed in 

 conjunction with the principal embryo, in the manner already described for the ventral edge 

 membranes. 



The dorsal edge membranes of " A " and " B " are separate in front, but unite posteriorly, where 

 they form one of the angles of the quadrangular tail fin. 



Alimentary Canal and Urinary Organs. The monster has two vents, which lie close together 

 just behind the pelvic fins of the principal embryo and of embryo " A." One of these leads into the 

 intestinal canal of the principal embryo, which is entirely separate from that of the others. The 

 second vent is the orifice of a portion of intestine common to " A " and " B." This common portion 

 bifurcates anteriorly near the middle of the small intestine, giving rise to separate stomachs, gullets, 

 air-sacs and livers, for the two embryos " A " and " B." 



There is a single urinary pore behind the vent. This pore leads into a bladder which is 

 common to all the embryos. The right Wolffian duct of the principal embryo has a separate 

 opening into the bladder ; the left Wolffian duct of the principal embryo and the right duct of 

 embryo " A " unite before opening into the bladder, as also do the left duct of " A " and the right 

 duct of " B." The left duct of " B " ends blindly. 



The alimentary and the urinary systems are illustrated in PI. XXII. figs. 95 and 96. 



