CTENOPHORES OF THE ATLANTIC COAST OF NORTH AMERICA 29 
ectoderm, while others lie at first at the lower (oral) pole of the embryo 
but are carried inward by invagination until they come to lie immediately 
under the apical sense-organ, where they give rise to the mesoderm. 
The stomodeum is formed by invaginated ectoderm derived from 
the original macromeres (figs. 26, 27, plate 6). The axial funnel-tube and 
peripheral chymiferous vessels are of entodermal origin and are formed 
from the macromeres after the formation of the primitive mesodermal 
cells. The body becomes egg-shaped, the aboral pole being broadest. 
There is considerable lateral compression, the tentacular axis being wider 
than the sagittal. When the animal is mature, however, the reverse is 
the case, for then the tentacular diameter is the narrower. The combs 
of cilia appear as 4 double rows of simple lashes. These soon separate, 
however, forming 8 rows of ciliated plates. The apical sense-organ is at 
first situated within a very shallow depression at the aboral pole, but 
this cleft gradually deepens until the organ becomes sunken deeply 
within a furrow. When about 30 hours old, the embryo has acquired 
4 double rows of cilia, a well-developed pair of lateral tentacles, and a 
large, apical sense-organ (fig. 34, plate 6). The entodermal part of the 
gastro-vascular system consists of 6 lateral diverticula from a central 
chamber; 2 of these lateral branches lead into the bases of the tentacles 
and the other 4 lead outward toward the 4 double rows of cilia. The 
ectodermal buccal pouch, or stomodeum, has become a long, laterally 
compressed tube, with its broad axis go° from the tentacular axis of the 
animal. Until this time the animal swims about quite freely within the 
egg-envelope, and even in this stage its cilia may be observed to beat in a 
normal manner and the tentacles to elongate or contract in response to 
stimuli. Soon after this the larva breaks through the egg-envelope and 
escapes into the water. Here it goes through stages which are so close 
to those of the young Pleurobrachia that it is almost impossible to dis- 
tinguish the embryos of the two species apart. The tentacles acquire 
numerous lateral filaments and elongate greatly, as in Pleurobrachia. 
When the animal is 5 mm. long the oral lobes begin to develop as two 
simple outgrowths on both sides of the mouth in the sagittal plane of 
the animal (see fig. 39, plate 6). The sagittal diameter is then somewhat 
longer than the tentacular diameter, and from this time onward the dis- 
parity between these two diameters increases at the expense of the 
tentacular diameter. At the time when the oral lobes begin to develop, 
the meridional ventral canals and the paragastric tubes begin to elongate 
downwards (fig. 39, plate 6). The former give rise to the characteristic 
loops in the oral lobes, while the 2 gastric tubes become J-shaped at 
their ends and finally extend up the lateral furrows which lie adjacent 
to the sides of the lobes, forming a circumoral vessel around the lobes. 
The 4 meridional vessels extend downward and fuse with the circum- 
oral vessel. The primary tentacle-bulbs migrate downward at the same 
time and come to lie close by the sides of the mouth, and they carry 
the tentacular canals down with them. The auricles appear last of all, 
after the lobes have developed to some extent. When 10 mm. in length 
the animal is ellipsoidal in outline and the condition of the lobes and 
auricles is similar to that in the adult of Bolinopsis (see fig. 40, plate 7). 
