34 Ech. 
ECHINODEKMATA. 
ingrowth of the ectoderm on each side of the stomach gives rise to the 
water pore and part of the water tube, meeting and uniting with a 
tubular outgrowth from the enterocoele. After the disappearance of the 
right water tube the left migrates towards the middle line. The paired 
pore canals show a resemblance to the spiracles of Appendicularia and 
tunicate larvae, in which also paired ectodermal involutions meet diver- 
ticula of the digestive tract. [ Vide Field.] 
Chadwick describes reproduction by fission in Cucumarict planci. The 
middle portion of the body became elongated and attenuated, and finally 
broke in two. The anterior half crawled away, and left the posterior 
part motionless. After about two weeks the posterior end had developed 
a neAv mouth and circlet of tentacles. This process occurred in three 
specimens, and the posterior half of one divided again, so that seven 
specimens were obtained from three. 
Cuenot (1) discusses the asexual reproduction of Echinoderms, pp. 
629-631. 
Driescii experimented on the segmenting ova of Echinus mici'otidier- 
culatus by shaking the eggs violently, so as to separate one of the first 
blastomeres. The remaining blastomere continued to segment, so as to 
form one-half of a normally segmented ovum. The hemispherical half 
embryo then closed up to form a blastula, with cells of the usual size, 
and normal in all respects except that it was smaller by half than the 
normally developed blastulse. From these blastulae were developed 
gastrulae, abnormal only in their small size. In many instances, when 
the shaking did not succeed in separating the blastomeres, twin embryos 
were formed, which was probably the direct result of injury, due to the 
shaking, on eggs which would otherwise have produced only a siugle 
embryo. [Cf. Fiedler.] 
Light was found to have no influence that could be observed, either on 
the segmentation or the formations of the organs. 
Fiedler experimented on segmenting ova of Ecltinoids by destroying 
some of the blastomeres, either by pricking them or by continued shaking 
up of them in a test tube. It was observed that the latter process caused 
the segmentation to go slower, and produced a certain pause for some 
timo. When ono of the two first segmentation spheres was slightly injured 
by a prick, so that some protoplasm, but not the nucleus, was lost, it was 
observed during the further segmentation that all the cells descended 
from the injured cell were smaller than those descended from the other, 
though otherwise normal. This difference was noticeable, even in the 
blastula, but after this disappeared. In other similar cases, where the 
nucleus also was lost, the injured blastomere always died. Most of the 
eggs operated on only ran through a few stages of segmentation, without 
reaching the blastula. The blastomeres surviving after the operation 
usually took on a spherical form, while those in uninjured eggs were 
elongate ovoids. After removal of one of the first blastomeres, the 
descendants of the remaining ones very frequently retained for a long 
time, not only their usual course of development, but also, so to speak, 
