680 The American Naturalist. [July, 
oral plates, the anterior being smaller and more sharply limited than 
the other. In twelve hours more a second groove appears behind the 
. first, cutting off a narrow ridge, the first post-oral somite. At this 
stage the embryo is readily comparable with Metschnikoff's Pl. XVII, - 
Fig. 3, except in the following particulars: The two ends of the 
embryo are more nearly equal, the single somite developed is much 
shorter, and the median groove is fainter and extends into both cephalic 
and caudal plates. Successive somites are added by budding from the 
caudal plate until the number six is reached. The embryo now closely 
resembles Balfour’s Pl. XIX, Figs. 32 and 34, except that it covers 
far less of the surface of the egg, the first somite is separate from the 
cephalic plate, the primitive groove extends across the somites, its 
anterior end terminating at the mouth, while posteriorly it runs into 
the caudal plate: the caudal plate is much smaller than in Balfour's 
figure. | 
(5) Just after six somites are formed, paired thickenings, the rudi- 
ments of legs, arise near the outer margins of each. Then six pairs 
arise simultaneously. I have seen no traces of Osborn’s semicircular 
groove. 
(6) Almost simultaneously with the outgrowth of the legs, paired 
thickenings for the nervous system appear. There are a pair of 
these in each somite of the body, while tbree pairs appear in the ceph- 
alic plate. A few days later a series of six pairs of segmentally 
arranged sensory thickenings arise outside of the legs, and extend in a 
line from the cephalic lobes backwards, as briefly described by Patten? 
These have different fates. The first pair gives rise to the median 
ocelli of the adult; the second to a peculiar sense organ as yet unde- 
scribed, occurring on the thin skin just in front of the first pair of 
appendages; the third soon disappeares ; the fourth forms the ‘ dorsal 
organ” of Watase, which persists longer than the third ; the fifth gives 
rise to the paired compound eyes; while the sixth is evanescent. At 
first these are all similar and are plainly sensory. These organs are 
connected with each other and with the brain by a longitudinal nerve, 
which takes an undulating course between the organs and the bases of 
the legs. 
(7) There is a precocious separation of ectoderm and entoderm 
(yolk cells) during the formation of the blastoderm. Blastopore and 
primitive groove produce no invagination of entoderm cells. The 
entoderm retains its primitive character as a solid mass of large yolk 
cells until after the caudal spine appears. The yolk cells are not true 
5 Jour. Morphology, 111. 
