ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
327 
Embryology of Earthworm.^ — Prof. E. B. Wilson gives a detailed 
account of bis observations on tbe embryology of tbe earthworms — 
Lumhricus terrestris, communis, and/a2^^d^^s. He finds that tbe cleavage 
is unequal and variable, and results in tbe formation of a blastula 
containing a large blastocoel. Tbe gastrula is formed by embolic invagina- 
tion. Tbe blastopore, wbicb at first occupies tbe entire ventral surface, 
narrows to a slit-like form, when its longer axis coincides with tbe long 
axis of tbe body ; it closes from behind forwards, but its foremost 
portion persists as tbe mouth. Tbe germ-bands are, from tbe first, 
united in the middle line behind tbe posterior lip of tbe blastopore, but 
remain separate in front until tbe establishment of tbe mouth, when 
they extend forward, join in tbe median dorsal line, and thus form a 
complete ring surrounding tbe region of tbe primitive blastopore. 
Tbe entire mesoblast is derived from a pair of primary teloblasts 
that lie at the j^osterior ends of tbe germ-bands, and no mesoblastic 
elements arise from tbe ectoblast wbicb overlies tbe germ-bands. Tbe 
primary teloblasts are differentiated in tbe course of tbe cleavage, and 
are pushed into tbe segmentation cavity some time before tbe com- 
mencement of gastrulation. Tbe cells formed by tbe continued pro- 
liferation of these primary cells are very early differentiated into two 
groups. Those of tbe first have histologically tbe character of meso- 
tbelium, form tbe mesoblastic parts of the germ-bands in tbe trunk- 
region, and inclose tbe paired coelomic cavities ; it is proposed to term 
them tbe trunk-mesoblast. Tbe cells of tbe second group arise by 
migration from tbe dorsal and anterior parts of the germ-bands, and 
may be called the migratory mesoblast. Histologically, they have tbe 
character of mesenchyme, and form a nearly complete investment of the 
body in tbe trunk region, but they also extend forward to form tbe 
cephalic mesoblast of tbe prostomium. 
When fully established tbe germ-bands consist, as in tbe Hirudinea, 
of three strata of cells : — an outer (ectoblast), one cell in thickness, which 
arises directly from tbe outer layer of tbe gastrula and persists as tbe 
bypodermis ; an inner layer (mesoblast) consisting of granular cells 
derived from tbe two primary teloblasts, and giving rise to muscles, 
septa, blood-vessels, peritoneal epithelium, reproductive organs, and tbe 
inner part of tbe nepbridia. Between these two there lies a layer wbicb 
agrees in general histological character with, and is indirectly derived 
from tbe former ; it gives rise to tbe nervous system, tbe outer part of 
tbe nepbridia, and tbe setigerous glands and setae. 
This middle stratum is arranged in a series of distinct longitudinal 
cell-rows, which, in early stages, lie at tbe surface and form part of tbe 
general ectoblast, but afterwards sink beneath and are covered by tbe 
rest. Until a comparatively late stage each row terminates behind in a 
large cell or teloblast which is tbe parent of tbe entire row, and thus of 
all tbe structures to wbicb tbe row gives rise ; they may be called 
neuroblast, nepbroblasts, and lateral teloblast, and may, collectively, be 
called tbe anterior teloblasts to distinguish them from tbe posterior or 
mesoblastic teloblasts wbicb lie at tbe extreme binder ends of tbe germ- 
bands. These anterior teloblasts first appear shortly after tbe com- 
pletion of gastrulation, when they lie at tbe surface. 
* Journal of Morphology, iii. (1889) pp. 387-462 (7 pis.). 
