1894.] Embryology. 77 
The reader of this chapter will notice how easy it is to extend ones 
anthropomorphism even to unclei! 
The noticeable fact brought out in studying the cleavage is that it 
is very irregular and variable in normal eggs. Even the third plane 
that we expect to find horizontal may be so only in a small part of its 
extent or, more often, quite vertical. These points will be considered 
in another paper by the author and Mr. Ecleshymer. 
The long axis of the animal is found to develope at right angles to 
the first cleavage plane: not agreeing with what seems the rule in the 
frog. This first plane cuts the egg at right angles to its elongation, (it 
is elongated by pressure in the oviduct) and this invariable rule is, the: 
author holds, the result of that pressure, the first spindle being able to 
place itself more readily in the one direction than in any other. He 
thus adopts, provisionally, a mechanical explanation of the determina- 
tion of the first cleavage plane in the newts’ eggs. 
In the vexed subject of gastrulation the author’s position is that in- 
vagination, to some extent, does actually occur. Epiboly can be act- 
ually seen in living eggs “the small cells roll down over the others 
(epibolic invagination), and at the same time the cells around the edge 
of the blastoderm turn in and dissappear from view (embolic invagin- 
ation?)” 
The blastoderm closes in first from all sides equally then, usually, 
more rapidly and equally from the right and left side, but sometimes 
from behind forward more rapidly, occasionally, perhaps, from before 
backword more rapidly. 
A slit is thus formed, which at first opens into the archenteron along 
its whole length but soon closes, except its anterior end, the evanescent 
neuropore, and its posterior end the definitive anus. 
The author restricts the term priniitive streak to the liner fusion of 
the germ layers. 
The conspicuous ectodermal furrow, “ neural groove,” is thus no part 
of the primitive streak: it may be- merely a result of mechanical 
stresses. 
The mesoblast is figured as presenting, in some cases, most seers 
lateral pouches at the anterior end of the embryo. 
Amphioxus.—Prof. E. B. Wilson‘ has published a fully illustrated 
account of the most important experimental work upon the eggs of 
Amphioxus mentioned in the January NATURALIST. 
Animal eggs show three chief types of cleavage the radial, the 
‘Journal of Morphology. VIII. August 1893. 
