u 
——- 
Normal Plates of the Development of Squalus acanthias. 75 
is from a stage between BaLrour’s “A” and “B” to and including Barrour’s “F”. (N.P.S. No. 3 
to N.P.S. No. 15.) The development of the medullary plate and groove is illustrated in a series 
of external views and transverse sections. No measurements of the embryos or counts of segments 
are given. 
In Locy’s final contribution!) (Table IX, column 9) a section is given to the description of some 
32 embryos ranging from a stage between Barrour’s “B” and “C” to approximately Barrour’s stage ER 
(N.P.S. No. 4 to N.P.S. No. 25). While the central nervous system and its segmentation receives the most 
attention a number of the anatomical characters of the earlier stages are described. Drawings of series of 
transverse sections accompany the descriptions of the embryos illustrated in Figs. 8, 7, 23 and 27. The 
supplementary description is not continued in detail for embryos older than Barrour’s “F”, although 
occasional mention is made of the number of somites, and the condition of the nerves, otocyst and gill- 
slits. Locy does not follow BarLrour’s classification of stages except in the case of his youngest embryo, 
which is listed as comparable with stage “B”. 
NEAL2), in his study of the segmentation of the nervous system (Table IX, column 9), worked upon 
a large series of embryos of Squalus acanthias. While his paper is in the main a consideration of the 
neuromeres, primary encephalic vesicles, and nerves, there are also many observations upon the head and 
trunk somites, gill slits, etc., and a large series of illustrations. The first six figures are of the external 
form of young embryos of from four to eleven somites. Figs. 7 to 21 inclusive are of the pharyngeal and 
cephalic regions of cleared specimens showing the outline of the brain, cephalic nerves, head somites, and 
gill slits. The nineteen stages represented by these figures together with another stage, a somewhat 
diagrammatic reconstruction of which is given in Fig. 40, are listed in Table IX. NEAL correlated three 
of his stages with those of BALFOUR as is shown in the table below. 
Babile\v. 
Tabulation of NEAL’s correlation of certain of his specimens of Squalus 
acanthias with BALFOURF’s stages. 
NEAL’s specimens 
BALFOUR’s stages 
Fig. Number of segments 
13 | 28—30 Early H 
17 50 } x 
19 65 
A section of Jomnston’s paper®) (Table IX, column 14), upon the morphology of the fore-braın 
vesicle in vertebrates contains a description of this structure in Squalus and a series of four plastic 
reconstructions which illustrate, besides the fore-brain, the structure of the pharynx, nerves and head 
somites. These stages represented by these reconstructions are approximated with BarLrour’s stages as 
follows: 
ı) W. A. Locy (1895), Contribution to the structure and development of the vertebrate head. Journ. Morph., Vol. 11, 
p. 497—586, 4 pls. 
2) H. C. NEAL (1898), The segmentation of the nervous system in Squalus acanthias. Bull. Mus. comp. Zool, Harvard 
College, Vol. 31, p. 147—294, Pls. I-IX. 
3) J. B. JOHNSTON (1909), The morphology of the fore-brain vesicle in vertebrates. Journ. comp. Neur. Psych., Vol. 19, 
p- 457—539, 45 figs. 
10* 
