696 SUMMARY OF CURRENT RESEARCHES RELATING- TO 
individualized units, it is very doubtful whether elements can migrate 
from the parablast to the blastodisc or embryo. The majority of the 
nuclei of the perivitelline membrane, after a period of indirect division, 
degenerate into chromatolysis. The function of the membrane is to 
elaborate the lecithin material in adaptation to the nutrition of the 
embryo. 
As to the germinal layers, the primitive entoblast divides into four 
portions : — (a) into a chordal portion which gives origin to the noto- 
chord, and (b) to the secondary entoblast, ( c ) into the elements of the 
mesoblast, and (d) into the residual or lateral entoblast. The inter- 
ruption between the chordal and lateral entoblast marks the communi- 
cation between the archenteron and the peritoneal cavity, where the 
mesoblastic folds have arisen by an abbreviation of that true evagi- 
nation seen in Amphioxus. But the greater part of the mesoderm 
arises directly from gastrulation, as a peristomial formation especially 
on the dorsal lip of the blastopore. 
Development of the Lamprey.* — He rr C. Kupffer publishes the first 
part of an extended memoir on the development of Petromyzon planeri. 
He accepts Bohm’s account of the fertilization of the eggs, and emphasizes 
the fact that the fertilized ovum and its segments exhibit two distinct 
portions, namely, active “protoplasm” free from yolk lying around the 
nucleus, and outside this passive “ paraplasm ” rich in yolk. Kupffer’s 
observations on the first steps in segmentation agree with those of Max 
Schultze. The time between fertilization and the hatching of the 
Ammocoetes varies markedly with the temperature, being seventeen 
days at Konigeberg but only eight at Naples. It may be divided into 
five periods : — (1) from the beginning of development till the definite 
separation of the central nervous system from the ectoderm ; (2) till the 
formation of optic vesicle and the auditory pit, and the segmentation 
of the mesoderm begin ; (3) till the nasal plate begins to be formed 
and the two anterior gill-pockets are recognizable ; (4) till the inva- 
gination of the mouth begins, when the gill-pockets are three in number, 
the auditory vesicle is constricted off from the epidermis, and cross- 
striped muscle-fibrils appear ; (5) from the invagination of the mouth 
till hatching. 
The cells of the most superficial layer of the morula unite into a 
blastodermic epithelium, beginning in an area on the future dorsal surface 
and spreading ventralwards. But before this is completed the blasto- 
pore is seen, so that gastrulation begins before the blastosphere is 
finished. A group of cells on the dorsal lip of blastopore, where 
ectoderm and endoderm are in contact, persists and increases, and 
determines the growth of the embryo in a caudal direction beyond the 
anus. This group is called the “ teloblast.” 
Kupffer then describes the “ keel,” in which the rudiments of central 
nervous system and notochord are combined, and shows how this becomes 
divided anteriorly into the paired head-ganglia, the brain, and the noto- 
chord. The formation of the central nervous system is peculiar, but it 
is not so divergent as it seems, for the dorsal portion of the keel which 
becomes the nervous system is not the product of a “ delamination ” 
Arch. f. Mikr. Anat., xxxv. (1890) pp. 469-558 (6 pis.). 
