22 DOUBLE MONSTROSITY STRUCTURE, CLASS VI. 



Spinal Cords. For the first three-fourths or so of the transitional region the spinal cords 

 are always wider apart than the notochords. This condition persists in cases where the notochords 

 unite in the heterocercal part, so that, here, union of the spinal cords is later than union of the 

 notochords. However, when the notochords come together earlier, i.e. in the neighbourhood of the 

 12th last body-segment, the spinal cords may approach each other suddenly towards the end of 

 the transitional region, in such a way that they finally unite with one another as early as the notochords. 

 In no instance of Class VI. (a) did the spinal cords or the notochords fail to unite ultimately. 



Intestine. The intestinal canals may unite anywhere within the last six or seven pre-anal 

 segments, hut do so usually quite close to the vent. As in previous classes the united portion has a 

 double dorsal mesentery. The vent itself is always single. 



Kidneys, etc. The Wolffian ducts, the bladders, and the urinary pores exhibit a number of 

 variations, and will most conveniently be described along with the corresponding organs, under 

 Class VI. (6). 



CLASS VI. (6). 



In the ventral union characteristic of this division, the tail shows four angles or keels which 

 are continued more or less regularly right to the end of the caudal fin. Of these keels, two (one 

 opposite to the other) are simply the dorsal edge membranes belonging to the mid-dorsal lines 

 of the component tails. The other two, also opposite to one another, are ventral edge membranes 

 and of composite derivation, half of each belonging to each twin. The manner in which these 

 composite edge membranes are formed may best be understood by supposing that the blastema 

 destined for the ventral portion of each embryo has been split into widely separated halves, and that 

 in the process of primary fusion two new wholes (of composite nature) have resulted from the union of 

 halves belonging to different embryos. A quadrangular tail with two (normal) dorsal and two 

 (composite) ventral edge membranes is shown in PL VII. fig. 30. A comparable result is seen 

 in the tail of the triple monstrosity, sections of which are shown in PL VII. figs. 32-4. 



There are two pairs of pelvic fins, i.e. a pair in front of each of the composite ventral edge 

 membranes on opposite sides of the quadrangular body. Obviously, each pair of fins is made up of 

 components which belong to different embryos. Usually one of the pairs is smaller and more 

 closely set than the other, and in such cases the vent, or pair of vents, opens behind the larger pair. 

 Occasionally what should be the smaller pair is deficient altogether. In the remarkable specimen 

 from which PL XIX. fig. 81 is taken, the two pairs of pelvic fins are exactly alike. This provides 

 an instance of perfectly symmetrical ventral union. 



Notochords. As regards the notochords, the rule is that they should unite for the whole, or for 

 part only, of their thinner terminal " heterocercal " portions, each of which tends naturally to bend 

 dorsalwards in its proper sagittal plane. When the portions in question unite at their commence- 

 ment, the two opposing tendencies to dorsal flexure neutralise one another so that the united portion 

 may project straight backwards. When union takes place near the extremity of the " heterocercal " 

 portions, a certain amount of divarication may always be noted before these bend round to meet one 

 another. This divarication is due to the fact that the tendency towards dorsal flexure has had scope 

 for some degree of independent manifestation in each of the notochords. My own material has not 

 furnished me with any examples in which the notochords never united, but instances of the kind 

 are stated to occur by Schmitt (%16). 



Spinal Cord. The spinal cords either unite the shortest possible distance in front of their 

 extreme posterior ends, or they never unite at all, and their union is typically later than that of the 

 notochords. 



Alimentary Canals. Three variations may be found : (1) Union occurring within the last three 

 or four pre-anal segments; (2) two separate vents opening side by side in front of one of the 

 composite ventral edge membranes; (3) two separate vents opening on opposite sides of the body, 

 one in front of each of the composite ventral edge membranes. In the first instance the dorsal 

 mesentery is double along the whole length of the united portion ; in the second instance the two 

 anal openings lie in a common field, and the last parts of the two recta are surrounded by a common 



