410 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
the yolk-sac of Sciuropsida and the typical mammalian blastodermic vesicle. 
There are 17 pairs of somites, which in the middle region of the trunk 
possess distinct cavities. Just external to the outer edges of the somites, 
with the exception of the first three, and between them and the lateral 
zone of mesoderm, there occurs a narrow intermediate zone, which con- 
tains the Anlage of the Wolffian body. At its posterior end this 
Anlage becomes reduced to a single cell, which passes directly over 
into the ectoderm. A distinct blastopore is present and leads into a 
canal which runs forward into the head-process of the primitive streak, 
and opens into the cavity of the vesicle. The embryo more nearly 
resembles that of the Virginian Opossum of 73 hours, as described by 
Selenka, than any other embryo known to the authors. The marked 
retardation in the formation of the medullary folds, and in the folding 
of the embryo, is one of the most characteristic features of the embryo 
at this stage. 
Gastrulatioii of Lacerta.* — Prof. L. Will continues his observations 
on the early stages of Eeptiles. The process of gastrulation in the 
Lizard appears to resemble that of other Eeptiles already examined by 
him. The primitive plate differs in form from that of the Gecko and 
the Tortoise, in being much less wide in form, and in not exhibiting the 
variations in length and breadth which are to be seen in the others. 
The terminal phase of the process of gastrulation consists in the forma- 
tion and the final disappearance of the primitive groove. In its formation 
both invagination and epiboly take part. The author thinks that his 
observations show that the homologue of the primitive stripe of the 
Amniota is to be sought for in the yolk-plug plus the lips of the blasto- 
pore in the Anamniota. In the Lizard the notochord arises, for its 
whole extent, from the upper wall of the archenteron. The mesoderm 
arises in exactly the same way as in the Gecko, with the exception 
perhaps of the region in front of the mesodermal plate. No part of the 
primary endoderm becomes definite enteric epithelium. The latter arises 
from the secondary endoderm. 
Experiments with Frogs’ Eggs.f — Mr. T. H. Morgan has been 
making experiments with the view of obtaining half embryos and whole 
embryos from one of the first two blastomeres of the Frog’s egg. These 
observations were undertaken because Mr. Morgan was of opinion that 
the results recently obtained by O. Schultze showed that the differences 
in the results of Eoux and Hertwig were owing to the difference in position 
taken by the living blastomere after the operation. In all he has ope- 
rated on 155 eggs, and from these he got eight embryos. From those 
eggs with a black pole turned upwards he got six half embryos, that is, 
embryos having only the right or left side of the body developed. These 
embryos are in every respect like those described by Eoux. In some of 
the eggs with a white pole turned upwards, the third horizontal cleavage 
appeared nearer the upper pole, so that the smaller white cells were cut 
off above. The two embryos which successfully developed were of half 
size, having both right and left medullary folds, and they are unquestion- 
ably perfect embryos of half size. The conclusion follows that the 
* SB. Ak. Wiss. Berlin, 1895, pp. 335-42. 
t Anat. Anzeig., x. (1895) pp. 623-8. 
