6 Oml 
XVI. CXELENTERATA. 
II. BIOLOGY. 
a. MORPHOLOGY. 
Hydrozoa. 
Bickford (18) ; Investigations made on Tubularia tenella confirm 
Loeb’s views on regeneration. Regeneration is not the result of budding 
or of new formation, but of the transference of stem tissue. If the 
stem is short, a partial hydranth is formed, which is subsequently com- 
pleted by growth. [See also Driesch (56).] 
Braem (26) corrects Lang’s statement that in Hydroidea buds are 
formed from one layer only. [See Lang (135), Brauer (27), and Seeli- 
ger (190).] 
Brooks (29) discusses the sense organs or cordyli of Laodice , in rela- 
tion to their development, physiological significance, and value from the 
point of view of classification. 
Bunting (35) describes the origin of the sex cells in Hydractinia and 
Podocoryne, and the development of Hydractinia. In Hy. the gonophor 
is a protrusion caused by the increase of cells, both in the ectoderm and 
endodorm. The ova appear to bo endodormal in origin, and to pass 
into the ectoderm. They do not always rest on the supporting lamella* 
The spermatozoa are ectodermal. The medusa-buds and generative 
products of P. are similar in origin to the gonophor of Hy. The ova 
of Hy. are laid at a definite period of the day. Polar bodies, two in 
number. Segmentation, total and equal. Endoderm formed by multi- 
polar delamination. Ccelenteric cavity, the result of a breaking down of 
endoderm cells. Tentacles are solid endodermal outgrowths, and the 
hydrorhiza is a stolon-like outgrowth which later becomes tubular. 
Chun (43) in an important paper on the laws of budding among the 
medusa), has arrived at the conclusion that the germinal layers have no 
previously determined peculiarities, either of a histological or organo- 
genetic character, and the only basis for an homology of the germinal 
layers is merely the relative position of those layers. The buds in the 
Margelidte have an ectodermal origin. 
Davenport (50) gives an account of his rogoneration experiments on 
Obelia commissar alis. The stoms of various individuals were cut at 
three different levels, and it was found that the whole stem, the hydranth, 
and a few rings only, may be reproduced after a cut at any one of the 
levels. The nearer the cut was to the proximal end of the stalk, the 
greater was the percentage of cases of reproduction of an entire stalk ; 
but the nearer the cut was to the distal end, the greater was the definite- 
ness of regeneration, the less was the mean number of rings regenerated, 
and the smaller were both the upper and lower limits in the number of 
rings regenerated. The facts are expressed in curves. The author con- 
cludes that the regenerative tissue is not differentiated at different levels 
for the purpose of producing different structures, but that it may pro- 
