NOTES ON THE EMBRYOLOGY OE LIMULUS. 
525 
chorion. From the time of this rupture until the assumption 
of a free life this larval cuticle functions, as Dr. Packard has 
aptly expressed it, as a “vicarious chorion.” As the embryo 
develops this cuticular envelope increases in size, always having 
a considerable space between it and the embryo (cf. Packard, 
’ 72 , pis. iv, v, figs. 19, 24, a). This increase continues until 
at last the “ vicarious chorion ” has about twice the original 
diameter of the egg. This increase in size is not due to any 
growth, but only to elasticity. In the later stages the pseudo- 
cells are no longer visible, but long before the final hatching 
they have twice their original diameter. 
A second cuticle is formed and molted before the embryo 
hatches. It makes its appearance at the stage 6, and in 
fig. 11 it is represented as lifted up on the extremities of 
the limbs. The active movements of the embryo soon tear 
this second cuticle off, and in the subsequent stages it is seen 
as a delicate membrane wadded up at one side of the chorion. 
Dr. Packard (’ 72 , p. 165) describes this molt. 
Between 4 and 5 a considerable gap occurs in my material, 
and I am not able to say whether here, as in scorpions (Metsch- 
nikoff, ’ 71 ) and spiders (Balfour, ’ 80 , b ), any metamerism 
appears before the appendages or not. In fig. 5 all the 
cephalothoracic appendages have appeared, and I have seen 
no evidence to support Dohrn’s idea that the first pair appears 
later than the others. Occasionally it is invisible in the living 
egg, but osmic acid always renders it distinct. An important 
fact was pointed out by Packard that at this early stage all 
the appendages are post-oral, although long before hatching 
the first pair acquires a pre-oral position. These appendages 
are at first simple outgrowths from the surface. In the 
median line of the embryo can be seen mouth and anus, each 
having a slender pyriform outline, the narrow end being in 
front. On first seeing this in connection with fig. 4, I was 
forcibly reminded of Balfour’s figures of Peripatus (’ 83 , pi. xx, 
figs. 34 to 37), and, were the narrower end of the mouth turned 
in the opposite direction, the natural inference would be that 
