216 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



yolk by long pseudopodia. The surface of the wall next the cavity is densely studded 

 with nuclei. This irregular cavity is undoubtedly formed by a folding of the embryonic 

 area, brought about by unequal growth, like the cases already described, and morpho- 

 logically lies outside the embryo. This is probably the same as the structure referred 

 to by Bumpus (30, p. 238). It has nothing to do primarily with either the endoderm 

 or the alimentary tract. 



It seems quite probable that many of the abnormal stages already described may 

 eventually attain to a normal growth and development, but this is not certain. 



DOUBLE MONSTERS IN OVUM AND LARVA. 



Brightwell, who gave a descrbption of the young of the European lobster (Romarus 

 gammarus), in 1835, was the first to notice double monsters in this species. He says: 

 "Two specimens of the young which appeared double were found, being strongly 

 united in the head " (24). In 1886 the first particular account of these monstrosities 

 was given by Ryder (171), who describes four types of fusion among the free-swimming 

 stages. It is to his kindness that I am indebted for the opportunity of examining the 

 six abnormal larvse which he described, two of which I have figured. 



It seemed worth while to trace, if possible, the history of these abnormal larv® 

 back to their early embryonic stages, but although I examined many eggs from many 

 individuals, I found only three monstrosities of this kind. The earliest is in the 

 invagination stage, corresponding to that shown on plate 53. It has the appearance of 

 a normal egg, except that instead of a single invagination there are two areas of 

 ingrowth. The axes of these two embryos appeared to be inclined to each other at 

 an angle of about 135°, and they were separated by considerably more than one-third 

 the circumference of the egg. The posterior ends of these embryos are the nearest 

 together, as with the others which I have observed. There is no cellular union of the 

 embryos in the yolk, but they meet in a common peripheral layer of cells, the nuclei 

 of which are now widely scattered, except in the immediate embryonic areas. 



The next youngest stage obtained was the egg-nauplius, which I have repro- 

 duced in fig. 235, plate 51. The two embryos are similar in every respect and lie almost 

 exactly opposite each other. The thoracic-abdominal plate of one embryo is joined to 

 that of the other by a long train of cells which extends through the yolk just below 

 the surface. These have the general character of migrating mesendodermic cells, 

 and some have already passed into the depths of the egg. 



In a little older double egg-nauplius the long axes of the two embryos make an 

 angle of about 160° with each other. Their posterior extremities are apposed and, 

 as in the first instance, separated by about one-third the circumference of the egg. 

 It is probable that had the two latter monsters been allowed to develop they would 

 have appeared, when ready to hatch, as if fused, back to back, like the fourth type 

 described by Ryder, in which four eyes were developed, two to each embryo, two 

 distinct sets of mouth parts, and biramous locomotor appendages. As Ryder remarks : 

 "This relation of two perfectly formed embryos in the same lobster egg is exactly the 

 reverse of that which is observed in vertebrates" (171). 



In the first type of larva described by Ryder there are no eyes; the cephalo- 

 thoraces are fused completely, both laterally and anteriorly, and the separate abdo- 

 mens diverge at a wide angle. In his second type (fig. 200) there is a single median 

 eye on the line of fusion of the cephalo-thoraces. The abdomens diverge at a very 

 wide angle, and, as seen in the drawing, there is a fusion of the first pair of antennae. 



