72 Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 
Two considerable gaps occur in BALFOUR’s series as figured. The period between stage “F” with 
the medullary groove widely open anteriorly, the cephalic plate widely expanded, and the incisura neurentica 
still in evidence, and “G”, in which the medullary canal is completely closed and the neck bend well 
established, corresponds to the largest gap. Stages “I” and “K” (there is no “J”) are also far removed. In the 
former the relation on the antero-posterior diameter of the yolk stalk to the length of the embryo is as one 
to four. In the latter the same relation is as one to seven. During the period which has intervened between 
these two stages the entire contour of the body has changed. The head is much more flexed and is increased 
in size. BALrouR states that embryo “L” is nearly twice as long as “K”, but considers that part of this 
difference in length is due to the fact that they are embryos of different species and genera and that the 
embryos of Sceyllium, of which “L” is one, are larger in proportion to the stage of development than are 
those of Pristiurus, of which “K” is an example. Embryos “E” and “F” are considered by BALFOUR as 
representing one stage. ‘“E” appears a little abnormal in not having a cephalic region as distinctly 
marked off from the trunk as usual.’ The figures of BaLrour’s series have been at least three times 
republished!) and two sets of embryos matched to the younger members of BALFOUR’s series have been 
figured and described 2). 
KASTSCHENKO?°) (Table IX, column 5) in 1888 in a preliminary contribution on the early development 
of Selachians presented a classification of the “stages of development up to the time when the medullary 
canal is completely closed”. No figures are included but the duration in days of several of the stages as 
observed at the aquarium at Naples is given in some instances. KASTSCHENKO’S classification, together with 
his correlation of his stages with those of BALFOUR, is given below in tabular form. 
Table I. 
Tabulation of KASTSCHENKO’s stages of Pristiurus melanopus, Seyllium catulus, S. canieula, together with 
their duration in time and their correlation with the stages of BALFOUR. 
KASTSCHENKO’s stages 
. Duration of stages in days wo 
No. Title of stages 
A*) B C 
I Befruchtete, aber unsegmentierte Keimscheibe . . . . . = = — = 
u OberllächlichesSeo.mentation a 2 = _ _ 
ITS SietesSeomentatonee ne ren m, 4 3—4 — _ 
IV Stadium der von außen sichtbaren Segmentationshöhle . 6—7 To IT Te u 
V Rüsselförmige Keimscheibe. .u. 2.73... Km Ina 29% 2—3 4 4 A 
VI allSatteHörmigerKreimscheiper. u ac en reE 3—5 4 5 _ 
VI AllkanzettenförmigersEmbryor er 2, Lab 2 2 — B 
Vor KnopförmigertEmbryor 2 A 1—2 2 _ D 
IX 7 'Stadiumsder;Medullärplattes, 2 2 see I = — = 
x Stadium.den, Mednllareinne 2 m... en 3 — —_ —_ 
X] |Stadium des eben geschlossenen Medullarrohres . . . . 2—3 _ = _ 
*) A Pristiurus melanopus, B Seyllium eatulus, C Seyllium canicula. 
I) a) 1896, B. DEAN, Fishes, recent and fossil. Vol. II. Columbia University Biological Series, Figs. 216—228, 284—289. 
(A figure of a large foetus of Squalus acanthias has been added to the series.) — b) I9o2, F. KEIBEL, Die Entwicklung der äußeren 
Körperform der Wirbeltierembryonen in HERTwıGs Handbuch der vergl. u. exp. Entw. d. Wirbeltiere, Bd. ı, Teil 2, Fig. 5, 
p. 15—22. — c) 1902, H. E. ZIEGLER, Lehrbuch der vergleichenden Entwicklungsgeschichte der niederen Wirbeltiere, Fig. 72, 
p- I0o2—105. (Figures of stages “B, C, E, H, M, P, Q” omitted.) 
2) a) 1892, H. E. ZIEGLER und F. ZIEGLER, Beiträge zur Entwicklungsgeschichte von Torpedo. Arch. f. mikr. Anat., 
Bd. 39, p. 56—ı02, Taf. III—-IV. — b) 1894, W. Hıs, Sonderung und Charakteristik der Entwicklungsstufen junger Selachier- 
embryonen. Arch. f. Anat. u. Phys., Anat. Abt., p. 337—354, Taf. XXI 
3) 1888, N. KASTSCHENKO, Zur Entwicklungsgeschichte des Selachierembryos. Anat. Anz., Bd. 3, p. 445—467. 
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