176 
MR. F. M. BALFOTJR ON THE DEVELOPMENT OF 
The nuclei are granular, and very often contain several especially large and deeply 
stained granules ; in other cases only one such is present, which may then be called a 
nucleolus. 
In sections there may be seen round the exterior of the neural tube a distinct 
hyaline membrane : this becomes stained of a brown colour with osmic acid, and purple 
or red with hsematoxylin or carmine respectively. Whether it is to be looked upon as 
a distinct membrane differentiated from the outermost portion of the protoplasm of 
the cells, or as a layer of albumen coagulated by the reagents applied, I am unable 
to decide for certain. It makes its appearance at a very early period, long before that 
now being considered ; and similar membranes are present around other organs as well 
as the neural tube. The membrane is at this stage perfectly continuous round the 
whole exterior of the neural tube as well on the dorsal surface as on the ventral. 
The section figured, whose features I am describing, belongs to the middle of the 
dorsal region. Anteriorly to this point the spinal cord becomes more elliptical in 
section, and the spinal canal more lanceolate ; posteriorly, on the other hand, the 
spinal canal and tube become more nearly circular in section. Immediately beneath 
the neural tube is situated the notochord (ch). It exhibits at this stage a central 
area rich in protoplasm, and a peripheral layer very poor in protoplasm ; externally 
it is invested by a distinct cuticular membrane. 
Beneath the notochord is a peculiar rod of cells, constricted from the top of the 
alimentary canal'*. On each side and below this are the two aortse, just commencing 
to be formed, and ventral to these is the alimentary canal. 
On each side of the body two muscle-plates are situated ; their upper ends reach 
about one third of the way up the sides of the neural tube. The two layers which 
together constitute the muscle-plates are at this stage perfectly continuous with the 
somatic and splanchnic layers of the mesoblast, and the space between the two layers 
is continuous with the body-cavity. In addition to the muscle-plates and their ventral 
continuations, there are no other mesoblast-cells to be seen. The absence of all meso- 
blastic cells dorsal to the superior extremities of the muscles is deserving of special 
notice. 
Very shortly after this period and, as a rule, before a third visceral cleft has become 
visible, the first traces of the spinal nerves make their appearance. 
First Stage . — The spinal nerves do not appear at the same time along the whole 
length of the spinal canal, but are formed first of all in the neck and subsequently at 
successive points posterior to this. 
Their mode of formation will be most easily understood by referring to Plate 16. 
figs. B l, B H, B in, which are representations of three sections taken from the same 
embryo. B i is from the region of the heart ; B n belongs to a part of the body 
posterior to this, and B hi to a still posterior region. 
* Vide Baifotje, “ Preliminary account of the Development of Elasmobranch Fishes,” Quart. Journ. of 
Microsc. Science, Oct. 1874, p. 33. 
