ages EAT 
1886. | Embryology. 743 
were fused together along their anterior dorsal aspects. There 
were four eyes developed, two to each embryo, and two distinct 
sets of mouth-parts and biramous, locomotive appendages. 
In this last form the two embryos adhere together dorsally, and 
€ two systems of limbs of the two nascent lobsters, which have 
obviously developed from a single blastoderm, enveloping one 
yolk, are directed away from each other. This relation of two 
perfectly formed embryos in the same lobster egg is exactly 
the reverse of that which is observed in vertebrates, For ex- 
ample, two embryo fishes which had been developed from the 
same blastoderm would, if primarily formed at exactly opposite 
sides of the blastodermic disk, finally be found to adhere together 
by their ventral aspects, so that their backs would look in oppo- 
site directions. These interesting data may in part be interpreted 
by the well-known fact that the neural aspect of an arthropod is 
ventral, while the neural aspect of a vertebrate is dorsal. This 
does not, however, afford a complete interpretation, since we find, 
not only in the case of a double vertebrate, but also in that of a 
similar invertebrate monster, that the median or unpaired organs 
of the two are often fused. This is the case in the double 
monsters of the lobster of the first, second and third types. In- 
asmuch as such structures as the alimentary canals and hearts of 
two such conjoined embryos are so fused, the question immedi- 
ately arises, How was this accomplished? The answer seems to 
be that the fusion took place coincidently with the process of 
gastrulation and the subsequent gradual formation of the organs 
of the two embryos. This view of the matter is further substan- 
tiated by the fact that the four types described represent as many 
degrees of fusion or union. The question which now arises, how- 
ever, relates to the way in which these different degrees of fusion 
of the two embryos were developed. 150 
is may be explained, it appears to the writer, on @ rior 
grounds, in the same way in which the different degrees of fusion 
or concrescénce of double fish embryos were interpreted by 
Rauber, and since verified by other observers. That is to say, if 
two gastrule were developed at the edge of the same blastodisk, 
