86 
ANNIE MATTHEWS. 
dermic elements, both being well developed in the ventral, 
while the ectodermic part is alone elaborated in the dorsal, 
14. Summary. 
(1) The fertilised eggs segment in various ways, but typical 
morulae always result. 
(2) When the sixteen cell stage again divides to produce 
the thirty-two celled embryo, delamination occurs, and from 
now onwards the larva is two-layered. 
(3) The morula at the twentieth hour begins to undergo a 
series of contortions which last from the first to the third day. 
This solid contorted stage is here termed the pre-planula, as 
it passes on into the hollow planula stage. 
(4) The pear-shaped planula, while swimming, exhibits 
-characteristic “ planarian-like ” movements, and on the fourth 
free-swimming day (the seventh day of development) it 
settles down by the broad anterior and aboral pole. 
(5) The settled larva soon flattens, assuming a mound-like 
$hape, and on the second day of fixation eight mesenteries 
grow out simultaneously into the ccelenteron from the base of 
the lateral wall. The ccelenteron is identical with the hollow 
central space in the endoderm of the planula. 
(6) The mesenteries, arranged in four distinct pairs, grow 
simultaneously and rapidly along the lateral walls and attached 
base, and soon nearly meet on the basal and oral surfaces of 
the polyp. 
(7) Many round cells now appear at the base of the columnar 
ectoderm. 
(8) Mesoglcea is at this time secreted by the endoderm, and 
flows round the above-mentioned round ectoderm cells, cutting 
them off either singly or in groups. These isolated ectoderm 
cells produce either nematocysts or spicules, the spicules 
appearing soon after the mesenteries. 
(9) Early on the third day eight simple hollow tentacles 
grow out, alternating with the mesenteries, and encircling the 
oral surface. 
