NOTE ON THE DEVELOPMENT OF AMPHIBIANS. 
299 
the beginning. The primarily broad space enclosed laterally 
between the medullary folds (M. F.) diminishes in size uutil it 
becomes a small rounded groove in the dorsal part of the 
neural rudiment, as represented at fig. 5 d. In a section 
through the cervical region of the same embryo (fig. 10 d) the 
epiblast has met above the groove, thus forming a relatively 
very small neural canal. As the epiblast of the two sides 
fuses above the canal the lumen of the latter becomes in some 
places suppressed, leaving as its only remnant a heavy accu- 
mulation of pigment. 
In the species of Frog examined by me the lumen of the 
neural canal at this period of development becomes suppressed 
throughout the posterior part, thus differing in this respect 
from Goette’s account of Bombinator. Towards its posterior 
extremity the neural rudiment of the Frog closely resembles 
that of an osseous fish at the same period. This manner of 
development of the neural rudiment in Amphibians presents a 
stage intermediate to the condition of Elasmobranchs and 
Reptiles on the one hand and the condition of Petromyzon and 
the Teleosts on the other. 
After the fusion of the epiblast dorsal to the neural rudiment 
the neural canal opens from before backwards aloug the 
pigmented line (</., fig. 5 n) which has previously been de- 
scribed. The canal, at first somewhat irregular, becomes in 
cross-section dorso-ventrally elongated. The walls of the 
neural tube become bilaterally symmetrical, and are thickest 
laterally. A transverse section through an older embryo of 
Amblystoma (fig. 11) shows the result of these changes. In 
the Frog embryo the appearance is fundamentally the same. 
In the cephalic region the development of the neural rudi- 
ment differs from that in the dorsal region. In order to clearly 
understand this difference, it is necessary to bear in mind that 
the anterior medullary plate (a. M. P., figs. 6 a, and 7 b) is 
not a paired continuation of the dorsal medullary plates, but 
is a transverse curved plate connecting the two dorsal plates. 
It is also necessary to distinguish the modifications caused by 
tlie cranial flexure, in order to recognise the homology between 
