170 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Development of Musculature in Birds and Mammals.* — Dr. A. 
Fiscliel finds tliat the primary or dorsal trunk musculature develops 
in all classes from the median myotome lamella ; the origin of the 
ventral trunk musculature and that of the extremities is more varied. 
In Selachii, Teleostei, Amphibians, and Reptiles it is processes of the 
ventral myotome-corners, the so-called muscle-buds. In Birds and 
Mammals it arises in the thick peripheral cellular mass of the Wolffian 
or appendicular ridge, which is no muscle-bud, but rather a mixture of 
cells, partly belonging to the somatopleure and partly arising from the 
myotomes. 
Development of Reptiles.l — Dr. L. Will, in the third of his con- 
tributions to the developmental history of Reptiles, deals with the 
formation of the germinal layers in the lizard. As is the case in other 
reptiles, the process of gastrulation consists in a close connection of 
epiboly and emboly. The former appears first, and leads to the laying 
down of the primary germinal layers and the primitive plate. The 
latter consists of an endodermic cell-material, which is in continuous 
connection with the lamella of endoderm. This last, therefore, forms 
at first a completely definite germinal layer, which at the end of the 
first stage divides into the primary endoderm or archenteric layer, and 
a secondary endoderm or yolk-layer. As in the Gecko, the latter 
extends as a simple cellular layer under the whole of the embryo. The 
yolk is also to be regarded as a part of the endoderm. Later, seg- 
mentation is effected by the yolk-nuclei, which exhibit mitotic division. 
To the epiboly, which introduced the process of gastrulation, the emboly 
is soon added, and brings about the invagination of the cell-material 
of the primitive plate and the formation of a hollow archenteron. 
It follows, from the author’s descriptions, that in the lizard the same 
structures are derived from the primary endoderm as in the other reptiles 
which he has already examined. The form of the primitive plate is from 
the first distinguished from that of the Gecko and tortoise, for the 
extended lamellar processes of the latter are altogether wanting. Other 
differences between the two are also indicated. The primitive plate 
becomes divided into two zones, the median area, which is homologous 
with the yolk-plug of the Amphibia, and a marginal area, which corre- 
sponds to the lips of the blastopore in Amphibians. The terminal phase 
of the process of gastrulation consists in the formation and the final 
disappearance of the primitive groove. The homologue of the primitive 
streak of the Amniota is to be sought for in the Anamniota in the yolk- 
plug, plus the lips of the blastopore. The mesoderm of the lizard, 
exhibits just the same relations as in the Gecko. The notochord arises- 
in the anterior part, from the anterior part of the archenteric region, 
from the axial division of the upper wall. The prostomial mesoderm 
arises by the metamorphosis of the whole prostomial archenteron, with 
the exception of the parts which are set aside for the notochord. The 
gastric mesoderm has its primary origin in the solid lateral wings of the 
archenteron ; with the exception, perhaps, of the region in front of the 
mesodermic plates, no part of the primary endoderm becomes definite 
enteric epithelium. 
* Morph ol. Jahrb., xxiii. (1895) pp. 544-61 (1 pi.). 
t Zool. JB. Abtli. Anat., ix. (1895) pp. 1-91. 
