50 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
(i. e. “ histogens,” descendants of blastomeres) enter, and fill up the 
lumen, and from these blastocytes the ectoderm is formed. The plastic 
blastomeres are thus divided into those which enter the brood-sac and 
form the ectoderm, and those which remain in the centre of the em- 
bryonic rudiment and form the endoderm. Thus, against Salensky 
and Brooks, Korotneff maintains the existence of two germinal layers 
in Salpa. A cleft between the endoderm elements is the beginning 
of the future respiratory cavity, the upper portion of which forms 
the cloaca, while a diverticulum becomes the pericardium. The heart 
arises as an invagination of the anterior pericardial wall. The nervous 
system is formed from a closed vesicle, which lies at first quite inde- 
pendently in the mesoderm, without any relation to the ectoderm or to 
the respiratory cavity, with which it subsequently becomes closely asso- 
ciated. As to the elaeoblast, it is wholly due to cells of the embryonic 
rudiment. 
Development of Synaseidise.* — Prof. W. Salensky, in his second 
communication on this subject, describes the development of Didemnum 
niveum , and sums up his general conclusions. 
A. The Development of Didemnidse and Diplosomidse. — The ovaries 
are without oviducts.. The mature ovum forms a protrusion on the 
integument, is surrounded by the latter, and constricted off into the 
cellulose cavity. 
The ovum is very rich in yolk, and is surrounded by kalymmocytes, 
by the follicular membrane, and by the maternal ectoderm. 
Segmentation is unequal, and results in an epibolic gastrula. The 
first two cleavages are meridional ; the third is equatorial, divides 
the egg into two halves of four cells each, and represents a frontal 
plane. The ventral group of cells is the rudiment of the ectoderm, the 
dorsal of the meso-endoderm. In Diplosoma the mesoderm is differen- 
tiated early in the form of two cells at the posterior end. These by 
division form the two symmetrical mesoderm-plates. 
The two mesoderm- plates represent the common rudiment of trunk 
and tail mesoderm ; the former falls into mesenchyme cells, the latter 
forms the caudal musculature. 
A nerve-plate appears dorsally at the posterior end ; plate becomes 
groove, groove becomes canal, and the system extends gradually forwards. 
The anterior neuropore remains open for a considerable time in Didem- 
num, but in neither of the types was a neurenteric canal observed. 
In front of the neuropore the neural canal ends in a blind sac, the 
rudiment of the primary funnel. To the right of this an expansion of 
the neural canal forms the sensory vesicle. The trunk portion of the 
neural canal thickens and loses its cavity ; the caudal portion remains 
thin. 
In embryos of Diplosoma a tubular process is formed on the right 
side of the neural canal — the lateral neural tube. It elongates ventrally 
and forms the rudiment of the nervous system of the ventral individual. 
It is absent in Didemnum. 
The primary funnel thickens posteriorly into the cerebral ganglion, 
the anterior part opens into the branchial sac as the infundibulum or 
secondary funnel. A chorioid fold is formed in the sensory vesicle, 
* MT. Zool. Stat. Neapel, xi. (1895) pp. 488-630 (4 pis. and 4 figs.). 
