ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
347 
longer than usual, and becomes elongated. It is succeeded by a form 
constricted in the middle into two vesicles — a thin “ gastrula-wall por- 
tion” and a thick “ enteron-portion,” which vary greatly in relative size. 
The constriction usually becomes complete, and by secondary growth a 
more or less narrow connecting portion is formed. The thick part fre- 
quently becomes divided into two. A ciliated band, homologous with 
that of the Pluteus, appears in varied form, and the development of 
the lithium larva ends. If less lithium be used, the type of larva which 
results is an exogastrula, i.e. with the endoderm part turned the wrong 
way, outwards instead of inwards, and it is this type which gives the key 
to the other. If the thick-walled vesicle correspond to endoderm, what 
takes place in many cases is a disproportionate growth of endoderm, in 
fact the series ends in a form in which the thin-walled vesicle (i.e. ecto- 
derm) has disappeared. This remarkable form Herbst calls a holoento- 
blastia ; it is distinguished in size, ciliation, structure and number of 
cells from an ordinary blastula or from Hert wig’s stereoblastulae, which 
are filled up with mesenchyme cells. The chief results, then, are two, 
the process of “ exogastrulation ” and an increase in the archenteric or 
endoderm-forming zone which leads to the two-vesicled type. 
Incompletely formed lithium larvae, returned to ordinary sea-water, 
continue their abnormal development, but not unless they have as blas- 
tulse remained 18-20 hours (at a temperature of 14-15° C.) in the 
lithium water. After a short stay in the lithium water the embryos may 
form a calcareous skeleton when returned to normal conditions. The 
effect of the lithium is less, the further advanced the stage at which the 
eggs come under its influence ; indeed typical lithium larvae are not 
certain to be formed unless the lithium has acted on the early seg- 
mentation stages. The first expression of the influence, however, is in 
the blastula, when the endoderm begins to be formed. 
Another interesting result of a short stay in the lithium water is 
the displacement of the Pluteus outgrowths of the larva. They may of 
course be entirely inhibited, but if the calcareous needles are formed and 
show displacement, the Pluteus outgrowths arise under their stimulus 
from groups of ectoderm cells which do not normally form them. This 
is against the doctrine of specific areas. So is it also with the ciliated 
band. The cells which ought to form it, don’t, but take on the characters 
of endoderm cells. Therefore, against Eoux and Weismann, the author 
maintains “ idioplasmatic equivalence ” of the cells in early stages. 
The author concludes with asking what kind of organism the lithium 
larvae would form if they grew up ? Would it illustrate “ saltatory 
evolution.” He points out, also, that each ovum has its individuality, 
each reacts differently, and the results form a graded series. Therefore 
the caution is suggested, that a gradual series of adults does not, in it- 
self, prove continuous descent ; “the series may have arisen because 
different individuals reacted in varying degrees to the same influences.” 
Herr Hs. Driesch* continues his investigation of the effect of artificial 
conditions on the development of sea-urchin eggs. 
His first chapter is entitled “ Exogastrula and Anenteria, or the 
influence of warmth on the development of Echinoid ova.” When blas- 
tulge of Sjphser echinus granularis, for about 26 hours after fertilization, 
* MT. Zool. Stat. NeapeJ, xi. (1893) pp. 221-54 (1 pi.). 
