ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
51 
abundant at Wood’s Holl in June, July, and August. The sperm 
mother-cell gives rise to four very small cells, each of which is, without 
further division, directly changed into the form characteristic of the 
spermatozoon. There seem to be no traces of the residual corpuscles 
which are supposed to be the homologues of the polar bodies. 
No special cell could be detected to which the origin of the mesen- 
chyme could be referred. The first traces of this tissue appear as 
soon as the ciliated coeloblastula becomes free-swimming ; in the region 
of the future endoderm, one and then more cells push out into the 
segmentation cavity and become amoeboid mesenchyme-cells. It appears 
to be certain that the development of Echinoderms is characterized by 
the absence of two bilaterally symmetrical primitive mesenchyme-cells. 
Asterias vulgaris seems to differ from Astropecten in that mesenchyme 
formation precedes and continues throughout the progress of invagi- 
nation. As in most other Echinoderms the mesoderm of Asterias 
originates partly as a mesenchyme formation and partly as an enterocoel 
formation, though there is not in this case a sharp morphological dis- 
tinction between them. Too much importance has been attached to the 
time of complete separation of the enterocoels, for it is subject to much 
individual variation. 
A right and a left water-pore appear, and, though some investigators 
have regarded this as pathological, there is reason to believe that it is 
a true ontogenetic character, and of very considerable phylogenetic 
significance. With regard to the significance of the larval form of 
Echinoderms it is well known that two very divergent views are held ; 
some regard it as having been cenogenetically acquired, others look 
on it as ancestral in character. Mr. Field points out that the ceno- 
genetic modifications are of little importance when compared with those 
which appear to be ancestral. The total and very nearly equal cleavage, 
and the ciliated blastula offer both simple ancestral conditions and means 
for wide distribution; the mode of mesenchyme formation is probably more 
primitive than the formation of the third germinal layer in the form of 
mesoblastic bands ; the derivation of this middle layer from any part 
whatever of the endoderm is antecedent to that condition where it is 
restricted to two special cells, the mesoblasts. The formation of 
enterocoels by archenteric diverticula is characteristic of ancestral forms, 
and in this larva are found the simplest conditions of complete enterocoels 
and archenteron, passing directly into corresponding parts of the adult. 
The bilaterally symmetrical water -pores cannot be supposed to be newly 
acquired characters, while the disappearance of the right pore may be 
explained by the subsequent connection between the two enterocoels by 
fusion in the preoral lobe. 
The Echinoderm larva is a form which has developed along the 
phylogenetic line, and is in many ways differentiated and capable of 
free existence : cenogenetic additions are transparency as a protective 
adaptation, and the formation of long arms for protection, but primarily 
as a means of increased locomotor power. The greatest of the 
cenogenetic modifications is that whereby the typical larva acquires 
the different forms characteristic of the various groups, but these have, 
since the time of Johannes Muller, been known to be all modifications 
of a single typical form. 
E 2 
