ae 
the all 
- SPOOND seRiEs, Vor, XXV, No 75.—MAY, 1858. 
45 
Agassiz on the Embryology of the Turtle. 353 
The germinal layer performs a triple office; in the 
‘Marrow 
median line of the embryo, it forms the basis of the. “ primitive 
tow” which is the incipient spinal marrow; exterior to this it 
constitutes the musculo-cutaneous layer; and more exteriorly 
still, it folds upwards over the back of the embryo, and becomes 
€ amniotic sac. 
The vertebral lamina is formed by the separation of a broad 
band, about equal in length with the embryonic disc, of loose, 
weonnected cells, from the upper surface of the subsidiary layer. 
From this the vertebra: are eveloped after the usual manner 
among vertebrates. The cells in the median line of the verte- 
il lamina become changed very soon, and more compact and 
unted, forming the chorda dorsalis, or the axis of the young 
vertebrae, which are not as yet apparent. By the development 
of the chorda dorsalis, the vertebral lamina becomes equally 
ided into two laminz lying right and left of the axis of the 
body. In about twelve days from this time, when four or five 
Vertebrae have appeared, and the spinal marrow 1s closed over in 
the anterior of the cerebral region, the subsidiary layer has de- 
Veloped a thick annular ridge on its under side all around the 
embryo, at a short distance beyond the confines of the original 
embryonic disc. This ridge eventually becomes a hollow mesh, 
© & sponge, and then constitutes the vena terminalis. Cotem- 
Poraneously with the last, another change occurs in the subsidi- 
ary layer by which it becomes separated along the median line 
of the ventral side of the body, from the vertebral laminse above 
t} and the heart and dorsal artery are hollowed out in its upper 
Side. Soon after this the omphalo-meseraic arteries are also hol- 
lowed out in the upper surface of the same layer from which the 
‘and dorsal artery originated. In these vessels a granular 
albuminous flyid surges backward and forward; thus evincing 
at this early period a beginning of blood circulation in vessels 
Which do not form.a complete circle with each other. It is very 
_ ““sily seen that the heart is the im elling power in this case. 
_the mode of formation and evelopment of the Wolffian 
bodies is so simple that it is a wonder there has been so muc: 
dispute among embryologists about their genesis. They ae 
48 mere thickenings of the subsidiary layer, around and “t ique- 
above each abdominal vein, and leaning over toward rn a 
artery, Being in the same layer with the arteries se h ae 
the blood vessels of the two systems meet in these organs throug 
_ Very simple channels, running as yet direct from the principal 
attery to the principal vein. About this time, when the heart 
has b come three-chambered, the vertebrae reach to the root of 
* tail, and the eyes have become entirely enclosed in complete 
orbits, antois begins to grow. Its formation is as simple 
