o4 QUARTERLY CHRONICLE OF MICROSCOPICAL SCIENCE. 
and rectum. From the sides of this central mass, at an 
early period, the hypoblastic cells arrange themselves so as 
to embrace the great mass of unorganized yelk in two diver- 
ticula of the middle intestine—the two liver sacs. This 
enclosure of the food-yelk by the hypoblast is remarkable in 
the animal series as not being constant, for it frequently 
happens, even in genera closely allied to those in which such 
an enclosure takes place, that the reverse of this disposition 
is carried out, so that diverticula of the middle intestine push 
their way into the mass of food-yelk, which lies then between 
them and the mesoblast, and apparently absorb it from the 
outside instead of from the inside after enclosure. The 
mesoblast in Oniscus, as in all other animal types possessing 
a body cavity (due allowance being made for the disputed 
case of Echinodermata), disposes itself so as to line the two 
limiting surfaces of that cavity—the tegumentary and the ali- 
mentary—furnishing muscles and skeleton. It also gives rise 
to vascular and circulatory organs by outgrowths into the 
body cavity. 
The Embryology of the Myriapoda is attacked by the inde- 
fatigable Metschnikoff in Kolliker und Sieb. Zeitschrift, 
part ii, 1874, under the title “‘ Embryology of the double- 
footed Myriapoda (Chilognatha).” ‘The observations refer to 
species of Strongylosoma, Polyxenus, Polydesmus, and 
Julus. They are the first studies of Myriapods’ develop- 
ment, carried on from the histogenetic point of view. New- 
port’s researches, which dealt only with external form, were 
the only observations up to this time. 
Metschnikoff says that he expected to find the amnion of 
Newport’s Julus the same as the insect-amnion, and a general 
agreement between Myriapods and Insects in their develop- 
ment. Instead of this he finds very great differences, as 
great as any between two groups of Arthropods. Firstly, in 
Myriapods there is a total cleavage of the yelk. This occurs 
in representatives of all Crustacean orders, more rarely in the 
Arachnida, but never in Insects. Thus the Myriapods agree 
with the Crustacea, but “it is to be observed,” says Metschni- 
koff, ‘* that there is no fundamental difference between the 
formation of the blastoderm in these groups.” Whether the 
plastic yelk is segmented or not, the cellular elements of 
the blastoderm are thrown off by it so as to form the super- 
ficial cell-layer. In many Insects the food-yelk cleaves after 
the separation of the blastoderm. Secondly, in the Myria- 
poda there are two germ-layers to be distinguished: an outer, 
agreeing in its differentiation with that of Scorpio, Araneids, 
Insects, and many Crustacea, since it forms the central 
