328 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
The prostomium is formed by the union of the anterior ends of the 
germ-bands ; the mesoblastic part arises by the forward growth and union 
in the median line of the mesoblastic bands — a process which is effected 
by proliferation and migration of the mesoblast cells already formed, and 
not by the formation of new mesoblastic elements from the superjacent 
ectoblast. The prostomial cavity is from the very first unpaired. The 
preoral or cephalic ganglia are differentiated out of the front ends of 
the neural rows, formed by the neuroblasts, at a time when these rows 
are fused with the ectoblast, and before they have met in the median 
line in front of the mouth. There is, therefore, no median apical plate, 
but only a pair of lateral ectoblastic thickenings continuous with the 
neural rows. 
As regards the mesoblast the author is fully convinced that the 
whole of the “ epiblastic mesoblast,” which Kleinenberg believed to bo 
derived directly from the outer layer, is simply the middle stratum 
which he failed to distinguish from the inner or mesoblastic stratum. 
This is a matter of importance owing to the views lately put forward 
by that author in his paper on Lopadorhynchus, and has some bearing 
on his denial of the existence of the mesoblast as a primary feature of 
development. 
Prof. Wilson thinks that the development of Lumhricus can be most 
simply and clearly interpreted in accordance with Sedgwick’s hypothesis : 
— (1) The ancestral form possessed an elongated ventral blastopore that 
gave rise to both mouth and anus by closure in the middle region ; 
(2) the mesoblast and the nervous system originally formed a ring around 
this blastopore, and subsequently underwent concrescence throughout 
its middle portion as the blastopore closed ; (3) the coelomic cavities 
were arranged in a continuous series in the mesoblastic ring, each 
lateral cavity lying opposite a corresponding cavity on the other side of 
the body, and a single anterior cavity lying in front of the mouth and 
giving rise to the head-cavity; (4) the larval trochosphere is secondarily 
derived from such a form by retardation or temporary suppression of 
the trunk-region, and early and extensive differentiation of the head- 
region. 
Embryology of Earthworm.* — Dr. R. S. Bergh has a preliminary 
notice of his recent studies on the development of the earthworm. He 
has particularly directed himself to an examination of the origin of 
the stripes of cells and primitive cells described by Wilson, but which 
Bergh failed to find in Criodrilus. In quite young embryos the number 
of such cells is very small. For example, in an almost spherical 
embryo, 0*125 mm. long, there are only two on either side; in an 
embryo 0*16 mm. long there were three primitive cells on either side, 
and it is only from the two inner on either side that quite short cell- 
rows arise. Later on the median of the three primitive cells divides 
into two, and all four begin to produce cells. It is, for various reasons, 
very probable that all the eight primitive cells arise from a single 
clea V age-sphere. 
The primitive cells, and the rows to which they give rise, are at 
first quite superficial in position, and the three primitive cells are to be 
* Zool. Anzeig., xiii. (1890) pp. 186-90. 
