DOUBLE MONSTROSITY DEVELOPMENT, ETC. 



E. DEVELOPMENT AND EAELY GKOWTH. 



It will be remembered that in the normal development of most fishes the segmentation of the 

 egg is partial, and leads to the formation of a cap-like or disc-like blastoderm, the edge of which 

 becomes thickened at one part and gives rise to the rudiment of the embryo. Gastrulation takes 

 place here, a " primitive streak " being formed, in part at least, by apposition of adjacent portions of 

 the blastodermic margin (Kopsch ; see further on p. 27). According to the concrescence theory 

 (associated particularly with the name of His), the body of the embryo is formed by the coming 

 together of the thickened margins of the blastoderm during the process by which this layer gradually 

 overgrows the yolk. 



The concrescence theory in its original form is no longer tenable, but the fact still remains that 

 the thickened margins of the blastoderm, close to the developing embryonic axis, are used up during 

 the early growth of the embryo, not in producing the axial organs, but in contributing material 

 for the formation of the muscle plates and the lateral and ventral body walls. This fact is of cardinal 

 importance in connection with what follows. A second point of great importance is that, while the 



blastoderm rapidly extends and grows over the 

 yolk, the extension sooner or later slackens 

 markedly in the region of the blastopore. 



The recorded observations indicate that 

 double monster fishes always arise on a single 

 yolk, and from a single blastoderm, at the margin 

 of which two more or less separate centres of 

 gastrulation (Hertwig 93a) and embryo-formation 

 have made their appearance. (We are leaving 

 mesodidymus and katadidymus out of count till 

 later (p. 25), because in fishes they constitute 

 types of peculiar and imperfect duplicity. What 

 is said on p. 28 regarding longitudinal or parallel 

 union should also be borne in mind.) 



The twin centres of embryo-formation men- 

 tioned above may be classed in two groups (a) 

 and (b), according to the distance which separates 

 them from one another, (a) In the first and most 



important group the interval is not too great to prevent approximation and union of the two 

 embryonic axes from taking place during the natural course of their growth in length (see Text- 

 figs. 1, 2 on this page). Approximation and union are due to the two factors to which attention 

 was directed above, namely, the utilisation during growth of the blastodermic margins near the 

 primitive streak, and the slowness of expansion on the part of the blastoderm over the yolk in 

 this same region. The twin adjacent axes are inevitably brought together posteriorly through 

 disappearance of the interval between them. The process may be called one of primary fusion, 

 in contrast with a process which often supervenes later, and which consists in the secondary fusion 

 of organs or structures already laid down. 



Primary fusion takes place earlier or later, i.e. in the head-, body-, or tail-region, according to 

 the interval which separated the embryonic rudiments when they first appeared. In other words, 

 the degree of duplicity varies directly with the original distance between the two centres of embryo- 

 formation. When the union is a purely lateral or an approximately lateral one, the posterior united 

 part finally becomes simply and perfectly bilateral. This takes place through the gradual fusion and 

 disappearance of structures belonging to the left and right halves respectively of the right and left 

 component embryos. Thereafter the right and left halves of the right and left embryos unite 

 naturally to form a normal bilateral body or tail. On the other hand, when the twin bodies come 



FIG. i. 



FIG. i. After Lereboullet (143 1863, PI. III. fig. 14). Egg of 

 the pike observed fifty-three hours after fertilisation, a, blastoderm 

 spreading over the yolk ; b, the portion of yolk not yet covered by 

 the blastoderm ; c ^, the two embryonic rudiments. 



FIG. 2. After Lereboullet (as above, fig. 15). Anterior part of 

 the same double embryo observed twenty-six hours later (at com- 

 mencement of fourth day), c c', the two heads showing optic 

 vesicles and uniting in the hind-brain region. Further back is seen 

 the commencement of the muscle somites. 



