476 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
formed in the region of the opening of the water-tube. The first pair 
of paired body-cavities do not originate as folds from the gut, but a 
proliferation of cells forms a thickened mass at two opposite areas of the 
mid-gut. These cells afterwards arrange themselves round a central 
cavity, and the body-cavity arises by increase in number of these cells ; 
the second pair of paired bodies arises from a solid fold at two opposite 
points of the posterior division of the mid-gut, which very early pinch 
off from the endoderm. 
The two eyes are not simple pigment-spots, but well-defined struc- 
tures; each is semicircular in shape, and each constituent cell ends 
towards the concave side in a sharp spine-like process. 
As the larva alters in shape there is a decided decrease in size and 
the ectoderm becomes thickened over the whole embryo ; the diminution 
in size is very similar to the process found in Echinoderms just before 
their metamorphosis. The most important change at this time is the 
development of the nervous system in the collar region ; a plate of 
ectoderm sinks below the surface, and at the same time the collar rolls 
over the invaginating plate of ectoderm from the two sides. It is clear 
that in this region the nervous system originates in the same way as in 
Amphioxus, that is, the ectoderm from the two sides rolls over a median 
plate and fuses above it. 
The walls of the heart are formed by the application of two vesicles — 
the mesenchymatous vesicle and the enterocoel— just as the other blood- 
vessels are formed by the contact of the body-cavities of the two sides. 
It is obvious that the similarity of Tornaria to the larvae of Echino- 
derms is very great, and the author cannot believe it to be superficial. 
If it be not, the antiquity of the larva must be very great. 
Echinodermata. 
Embryology of Asterias vulgaris.* — Mr. G. W. Field has commenced 
the study of the development of the common American starfish. The 
mother-cell gives rise to four spermatozoa. The formation of mesen- 
chyme appears to precede and to be continued during the process of 
invagination ; cells in the endodermic region of the blastula divide 
transversely, the part next to the segmentation cavity becomes amoeboid, 
and wanders freely in the jelly-like substance, filling the segmentation 
cavity. Any endodermal cell may, with or without division, become a 
free, wandering, mesenchyme-cell. These observations seem to confirm 
the view of Metschnikoff and Korschelt as to the absence of the two 
primitive mesenchyme cells in Echinoderms. 
The amoeboid mesenchyme-cells form a supporting network between 
the external walls and the digestive tract ; many apply themselves to 
the wall of the body and of the digestive tract ; on the former they give 
rise to a discontinuous lining, and on the latter they form long, delicate, 
anastomosing processes, and give rise to muscles. The author’s account 
of the distribution of cilia does not quite agree with that of Semon. 
There is an ectodermal thickening at the apex of the pre-oral lobe, due 
to the more columnar character of the cells in this region ; this thicken- 
ing corresponds exactly in position with the apical plate of Tornaria. 
* John Hopkins Univ. Circ., x. (1891) pp 101-3. 
