440 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
When the embryonic shield has reached its fall size, the primitive 
endoderm is composed of two layers of cells distinctly marked off, except 
in the middle line, where there is a fusion. The streak of fused cells 
presently acquires a sharper lateral boundary, and becomes the noto- 
chord. On each side the under of the two lateral layers grows beneath 
the notochord and, uniting with its fellow, forms the endoderm proper. 
The upper lateral plate thickens by cell-division and forms on each side 
the mesoderm plate. In the extreme anterior region the whole of the 
primitive endoderm becomes transformed into the mesoderm of the head. 
The alimentary canal is formed by a process of folding, essentially 
similar to that found in the Amniota. Kuppfer’s vesicle is not formed 
in a manner essentially different from that of the rest of the alimentary 
canal. The peculiar features of the vesicle are the size and early 
appearance of the fold, and (in pelagic eggs at least) the fact that the 
periblast is here pushed down. The characters of this vesicle are dis- 
cussed at considerable length. 
The Wolffian duct arises as a fold of the body-cavity, and at no time 
has any connection with' the ectoderm ; Brook, in the author’s opinion, 
probably mistook for it the rudiment of the lateral line. 
The development of the sensory organs and of the lateral line is next 
described, and the history seems to show that the superficial sensory 
patch found in larval fish does not represent the condition of the primi- 
tive segmental or branchial sense-organ. The author’s account of the 
development of the lateral-line organs is radically different from that of 
Hoffmann, and lends no support to Eisig’s view of the homology between 
the lateral-line organs of fishes and those of certain Annelids. 
Karyokinesis and Cleavage of Ovum.* — Mr. S. Watase agrees with 
Van Beneden and Boveri in holding that the achromatic spindle plays 
the most important part in the production of the karyokinetic pheno- 
menon. It is the mechanism by which the chromatic substance of the 
spindle is divided among the daughter-cells. But he cannot find any 
evidence of the contractility of the achromatic fibrils ; on the other 
hand, he finds that the achromatic threads are constantly lengthening, 
stretching, and pushing away from the centres of the asters from which 
they start. The achromatic spindle in its perfected form consists of 
two cones with their bases turned towards each other, with a sheet, as it 
were, of the achromatic substance of the nucleus interposed between 
them. Each cone is a part of a more general system of radiating fibrils 
forming one of the asters. The asters in a cell arise from the preceding 
single aster, as the new nuclei arise from the preceding nucleus. The 
old aster divides into two, each daughter-aster having a granular sub- 
stance in its centre, and around it the achromatic rays extending in all 
directions. As the rays from each of the small asters grow longer the 
centres of the corresponding asters become more and more widely sepa- 
rated from one another. A small achromatic spindle is formed by the 
two groups of achromatic rays between the two centres. When the two 
asters become so widely separated as to have the whole nucleus between 
them, they apparently come to rest and begin their work on the nucleus 
by pressing on the more solid portion of the nuclear contents. The 
* John Hopkins Univ. Circ. ix. (1890) pp. 53-6. 
