MUSEUM OF COMPARATIVE ZOOLOGY. 73 
cephalic plate. The addition of new zonites to those already existing 
goes on rapidly ; the two anterior ones (those of the cheliceree and the 
pedipalpi) are cut off from the posterior end of the cephalic plate. They 
are late in making their appearance, and, as Balfour puts it, ‘‘lag behind” 
the others in their development. The other zonites are developed from 
the caudal plate. 
Soon after the protozonites are first established they form ridges which 
reach nearly around the egg, and thus appear to radiate from the dorsal 
region. (Compare Emerton, ’72, Figs. 8, 9.) They soon undergo con- 
centration which so shortens the thickened ridges, that together they 
form a band about 45° wide on the ventral surface of the egg — the 
embryonic band. Fig. 6, Pl. II., gives a side view of an egg in which 
this concentration is well advanced but not yet completed. At the same 
time the embryonic band increases in length, thus extending in an antero- 
posterior direction further and further around the egg. When at length 
seven or eight protozonites are fully established, the band embraces 
approximately two-thirds of its circumference. At about this time also 
the rudimentary appendages begin to appear; these mark the commence- 
ment of the third period of growth. 
The internal condition of the egg during the second period can be 
satisfactorily studied only by means of sections. I have made sections 
passing through the primitive cumulus in two directions, sagittal and 
transverse. In sagittal sections two features are conspicuous: (1) The 
ventral surface of the egg is clearly differentiated from the dorsal surface 
by the condition of the cells along its entire length (Pl. VII. fig. 41). 
(2) The cells in the region of the cumulus are arranged in several irregu- 
lar layers. A thickening of the blastoderm has also arisen at the caudal 
eminence, and there is a tendency to thicken along the ventral region 
embraced between these two structures. 
Figure 41 is from a sagittal section of the egg represented ‘in PI. I, 
Fig. 3; the cells of the ventral side are large and rounded or oval, 
while those of the dorsal side are much flattened. The cells of the 
primitive cumulus (cum. pr.) are conspicuous for their size; they are 
loosely arranged in layers. In some cases (Pl. VI. fig. 39) they are four 
layers deep. 
Sections of eggs a little more advanced show a large number of cells 
along the ventral-plate region, and also at the caudal thickening. 
Balfour’s figure (1. ¢., Fig. 11) of this stage cannot be compared criti- 
cally with my own, as he was uncertain about the direction in which the 
egg was cut; but from its close resemblance to my sections, I think it 
