ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
345 
mid-brains, and it is in this region also that the nervous tube first 
becomes separated from the epidermis. The epidermis, when histo- 
logically differentiated, forms a kind of groove, the walls of which form 
an angle into the nerve-tube ; the walls of this last gradually fuse 
towards the periphery, and little by little push out the epidermic groove. 
This accomplished, the nerve-tube becomes completely separated, and 
the groove is converted into a thickening ; as there is here no separate 
morphological formation, there can be no question as to the formation of 
the foundation of the peripheral nervous system. 
This latter system is differentiated thus ; the foundation of it appears 
after the closing of the nerve-tube, and its separation from the ectoderm ; 
it arises in the upper wall of the tube in consequence of the multipli- 
cation of the elements of the tube, which alter in position as they divide ; 
the further growth of the system depends on the independent multipli- 
cation of its elements. It appears first in the region of the parietal 
inflexion, and extends forwards as far as the neuropore, and gradually 
becomes differentiated posteriorly. The part first to become separated is 
the nervous group in front of the trigeminal, and then follow others, so 
that five nerve-groups are for a long time in connection. 
In the Sauropsida the differentiation of the foundations of the peri- 
pheral nervous system is accomplished before the nerve-tube closes and 
separates from the ectoderm ; when the tube closes the foundations are 
found in the space between the tube and the ectoderm. Other differences 
from the Selachians, which may be taken as typical of the Ichthyopsida, 
are pointed out, and may be explained as due to the general differences 
in the development of these two great groups. 
We may conclude that the peripheral nervous system is typically 
developed in direct dependence on the central system ; the ectoderm 
itself takes no part in its formation ; in the body the development of the 
spinal nerves exhibits primitive characters in all Mammals ; there is, for 
the whole of the peripheral nervous system, a general germ which is 
gradually developed from before backwards, and is afterwards dislocated ; 
this dislocation may be considerable in the anterior region before the 
germ has appeared further back ; in this case the Selachians exhibit 
primitive characters. 
Development of Segmentation-cavity, Archenteron, Germinal 
Layers and Amnion of Mammals.* — Dr. A. Robinson commences with 
a general description of the development of the ova of the rat and mouse 
up till the period of the completion of the blastodermic vesicle ; he finds 
that there is a segmentation-cavity which is not the blastodermic; it 
disappears at the time when the archenteron, which is developed amidst 
the hypoblast, appears. The young ovum consists principally of hypo- 
blast, which becomes vacuolated to f ,rm the cavity of the yolk-sac. 
The cavity of this sac is never bounded by epiblast alone ; this layer 
extends over the outer surface of the hypoblast, but the latter is never 
entirely surrounded by epiblast. 
The mesoblast is described as being formed partly from the peri- 
stomial cells in the region of the primitive streak, partly from the 
embryonic hypoblast, and partly from the extra-embryonic hypoblast. 
* Quart. Journ. Micr. Sci., xxxiii. (1893) pp. 369-455 (5 pis.). 
