ON THE AGAMIC REPRODUCTION AND MORPHOLOGY OF APHIS 69 
increased growth of the body as compared with the head, the com- 
pletion of the dorsal surface by the upward extension of the blasto- 
derm, and the gradual restriction of the yelk to the anterior part of 
the body. 
I have been unable to determine, as precisely as in Apfzs, the 
exact share taken by the procephalic lobes in the composition of the 
roof of the head in the crustacean ; but they assuredly extend over a 
considerable part of its latero-dorsal parietes. 
The carapace appears at first as a ridge-like process developed 
from the dorso-lateral region of the antepenultimate and preceding 
thoracic and cephalic somites, as far forwards as the bases of the 
antennules. It is certainly not an extension backwards of the terga 
of any of the anterior cephalic somites, but is from the first continuous 
with, and developed from, the thoracic somites. 
It is needless to trace the history of the larval AZyszs further,— 
what has been said sufficiently proving the close resemblance of its 
development to that of Aphis. 
§ 3. Embryogeny of Scorpio as exemplifying Arachnida. 
I have not yet had the opportunity of working out the develop- 
ment of an Arachnidan ; but the researches of Rathke! and Herold? 
are so full and clear, that the omission is of little moment. 
Rathke’s observations on the development of the Scorpion show 
that after, or even before, the blastoderm has extended over the whole 
yelk, a papillary elevation appears at one pole. It is the rudiment of 
the future abdomen, including under that term all the segments of the 
body behind that which carries the last pair of respiratory organs. 
In front of this, eleven pairs of closely approximated thickenings 
make their appearance; and then, at the sides of the sixth to the 
tenth pair of them, inclusively, counting from the rudimentary abdo- 
men, papillary processes are developed. It is clear, from Rathke’s 
figures, that the anterior pair of thickenings are the “ procephalic 
lobes,” while the succeeding ones are the sterna of the somites 
between the mouth and the abdomen. The five pairs of processes 
thrown out by the five anterior of these are the great chelz and the 
four pairs of ambulatory appendages. The antenne make their 
appearance subsequently from the procephalic lobes (or their junction 
with the rest of the blastoderm) in front of the mouth. It is not 
expressly stated, but I do not doubt, from Rathke’s figures, that the 
1 Reisebemerkungen aus Taurien, 1837. 2 De Generatione Aranearum, 1824. 
