No. 2.] ANURIDA MARITIMA. 267 
because it is not arule that the smallest is the least developed. 
Quite often the smallest ones have undergone considerable 
post-embryonic development, while some larger ones are much 
farther back in the process. Pigmentation and increase in 
size are the chief external changes that are needed to make 
the young Anurida resemble the adult. Both of these charac- 
ters come slowly, though the young are probably all pigmented 
by the end of the season. They remain small in size, however. 
Comparing these results with Ryder’s (’86) figures, which 
are, as far as known, the only published studies of the em- 
bryonic stages of Anurida, certain differences are observable. 
There are figured in these the two crenated membranes and 
the early stages of the germ band, several later embryonic 
forms, and young and adult animals. The chief difference in 
the embryos as figured by Ryder and those shown in Pl. XXIII 
of this investigation lies in the different interpretation of the 
embryonic head appendages. Ryder recognized but three 
pairs: one pair of antennae, one pair of mandibles, and one 
pair of maxillae. He included the second maxillae with the 
thoracic legs, and did not see the intercalary appendages. In 
the recently hatched young he describes a structure placed on 
the anterior part of the fourth abdominal segment which he con- 
siders represents a rudimentary spring. No evidence of such 
a structure was seen in the young investigated, and large num- 
bers were examined. In the embryonic stages the appendages 
on the fourth abdominal segment are larger than any of the 
others, excepting those on the first (Fig. 45, a.4); but these, like 
all the others excepting those on the first, disappear before 
hatching. 
The process by which the germ band arises is exceedingly 
simple. Immediately after the formation of the second cre- 
nated membrane, or even before, or in some cases before all 
the entodermic nuclei have sunk to a common level again, the 
mesoderm cells may be seen migrating to such a position that 
one meridian passing through the precephalic organ and the 
centre of the egg would cut the band they form longitudinally 
into two. The migration eventually leaves the greater part of 
the egg covered only by ectoderm and the germ band appears 
