166 The American Naturalist. [February, 
trophoblast cells of Hubrecht, which are ectodermal. Hubrecht shows 
conclusively that the endoderm originates from the embryonic knob, 
and not at its sides as is demanded by Minot’s recent hypothesis. 
Hubrecht is more in accordance with Haddon, both as to the origin * 
of the endodern from the under side of the embryonic shield, and in 
the ectodermal covering of the the early blastocyst.—T. H. M. 
The Embryology, of Gecko.—Dr. Ludwig Will gives, in the 
- Biologisches Centralblatt, November 15, 1890, a short paper on the 
method of gastrulation of this lizard. ‘At the posterior end of the 
embryonic shield is a mass of cells, called the primitive plate. The 
cells at this point are several rows deep, while over the embryonic 
shield the ectoderm is composed of a single layer of columnar cells, 
but witha few yolk-cells scattered beneath it. At a later stage the an- 
terior end of the primitive plate forms a distinct invagination, the walls 
formed of a single row of cells. This sac pushes forward under the 
embryonic shield, between the ectoderm and the yolk-cells, The in- 
vagination cells spread out into a broad sac. There follows next an 
irregular fusion and absorbtion between the invaginated endoderm and 
the yolk-cells (endoderm also), so that the general cavity above the 
yolk, in which the yolk-cells were scattered, communicates with the in- 
vagination cavity, and hence with the outer world by means of the 
proximal end of the latter cavity, or blastopore. The upper walls of the 
invaginated cells go to form the notochord, and the rest of them go to 
form the mesoderm at the sides of the latter, The author believes that 
the Gecko furnishes grounds for comparing the reptilian with the am- 
phibian gastrulation. The blastopore—or the open mouth of the in- 
vagination—extends backwards, and the two lips coming in contact 
fuse to form a primitive streak, so that what was previously only a theory 
—namely, that the primitive streak was formed by the fusion of the lips 
of the blastopore in Sauropsida, and whose opening in these was only 
represented by the neurenteric canal—is now shown to be a fact from 
the development of the Gecko. 
Theory of the Mesoderm.’—Prof. C. Rabl has a long paper on 
the origin of the mesoderm of vertebrates. The paper is largely de- 
voted to theoretical discussions, although based upon observations on 
the germ-layers of Selachians, birds, and mammals. The first part of 
the paper deals with the formation of the mesoderm in the above types, 
the second with the later differentiations of the mesoderm. It is un- 
necessary to give a full review of the paper here,? and we may confine 
5 Morphologisches Jahrbuch, No. 15, 1889. 
See Journ. Royal. Micro. Soc., Feb., 1890. 
