ON THE AGAMIC REPRODUCTION AND MORPHOLOGY OF APHIS 7I 
3. In the Articulate embryo, therefore, the neural wall is formed 
first, and gradually extends tergally so as to form the hemal wall. 
4. The cephalic blastoderm very early undergoes a_ peculiar 
flexure, a greater or less portion in front of the mandibles being 
bent up at right angles to the rest, and even in many instances 
extending backwards, so as to constitute the entire hamal region 
of the head. In these cases the top of the head is in reality a 
sternal, and not a tergal surface. 
As a consequence of this flexure, the line of attachment of the 
bases of the eyes and antenne is frequently altogether above that 
of the other appendages, so that they appear to be tergal, and not 
sternal, appendages. 
5. The anterior extremity of the cephalic blastoderm becomes 
early divided by a median fissure, each lateral portion being a 
“procephalic lobe.” In Insects the line of junction of these pro- 
cephalic lobes is the epicranial suture. 
6. In Jnsecta and Crustacea the head, in the embryo, is easily 
distinguishable from the rest of the body. In Podophthalmous 
Crustacea it is clearly seen to be composed of six somites, each 
possessing a pair of appendages; of these, the first are the eyes; 
the second, the antennules; the third, the antenne; the fourth, the 
mandibles; the fifth, the first maxilla; and the sixth, the second 
maxillze. 
In Jusecta, on the other hand, only four pairs of appendages 
appear in the head, the eyes being sessile, and one pair of antennary 
organs remaining undeveloped. 
In the Arachnida it appears to me to be quite clearly shown: by 
development that the anterior pair of appendages are antenne ; the 
second pair, mandibles, with a hugely developed palpus; the third 
pair, first maxilla; and the fourth pair, second maxilla, converted, 
like the next two pairs of appendages, into ambulatory legs. 
It follows, therefore, if we take the number of moveable ap- 
pendages as the test, that in the Avréeculata never more than six, 
and never fewer than four, somites enter into the composition of the 
head. But is the number of moveable appendages a just test of the 
number of somites entering into a part? No one will pretend that 
it is so in the abdominal and thoracic regions; and if we consider 
the head of Crustacea alone, we find the eyes becoming sessile, and 
one pair of antennary organs aborting, without the least reason for 
concluding that the typical structure of the head is altered. It seems 
to me, then, hardly a hypothesis to assume that the sessile eyes of 
Insects represent the appendages of a somite, since it is universally 
