220 ; BUNTING. [Vou. IX. 
delamination. This method of origin of the endoderm has 
lately been noted by Brauer for Hydra, where he finds multi- 
polar delamination, mixed with an inwandering of cells. By 
examining the next section in the series, however, the spindle 
is always found, which demonstrates that there is simply an 
elongation prior to delamination (Fig. 55,4). The endodermal 
cells divide karyokinetically, as well as the blastula cells, until 
we have a solid mass of cells formed. This consists of two 
layers, an external one-celled layer and an internal mass of 
cells (Fig. 56). 
FURTHER DEVELOPMENT OF THE EMBRYO. 
At about twenty-three hours, we find the endoderm beginning 
to hollow out (Fig. 57), although it is still several cells in 
depth. The ectoderm is composed of a single row of columnar 
cells intermingled with numerous gland cells. There is a very 
sharp outline formed between these two layers, which gradually 
increases, forming the so-called “supporting lamella.” Longi- 
tudinal sections show that this cavity, which has arisen by the 
breaking down of the endodermal cells, has, as yet, no con- 
nection with the exterior. Also we find that the embryo has 
elongated. The digestive cavity remains closed to the exterior 
for several days. Fig. 58 shows a section of a planula which 
has been attached at point A, for four and one half days; the 
coelenteric cavity C, is still closed. 
The mouth pole assumes the conical shape of the hypostome, 
while the tentacles appear as outpushings of both ectoderm and 
endoderm (Fig. 59). A step still farther in the development 
is shown in Figs. 60, 61, both of the same hydroid in which 
the coelenteric cavity has opened to the exterior. The ten- 
tacles are solid, consisting of a layer of ectoderm surrounding a 
single row of endoderm cells. The cavity of the hydrorhiza 
(D) is found to be in connection with the main digestive cavity. 
The hydrorhiza forms the coenosarcal tubes which connect the 
different members of the colony ; and later, a secretion from 
this coenosarcal base forms the chitinous ground work, with its 
many protruding spines, 
