268 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
its fuiictiou. The causes which led to the loss are plain enough. The yolk sac re- 
mains very large long after active muscular movements begin (movements begin 
before hatching) and a ventral musculature beneath the yolk (which would occupy the 
position indicated by dotted line in Fig. 10, p. 264, an. v. m.) would consequently be of 
no service dntil late in larval life, when the sac should have disappeared. This being 
the case, a much more economical method of forming the mesoderm was to put a stop 
to the subvitelline ingrowth (an. v. m.) and allow the lateral plates to form the ventral 
as well as dorsal mesoblastic tissues. The ventral (subvitelline) mesoderm, having in 
this way lost its function in the Teleost, must be regarded as a rudimentary organ of 
the gastrula. It always remains very small, and does not form any special organ or 
set of organs in the embryo. Being present it is however made use of, and goes to 
form a mass of indifferent material (caudal mass) at the expense of which the organs 
in the tail develop. 
The germ ring, in toto, according to the view which has just been given, is not a 
peculiarity of the Teleost. It is a feature which the Teleost gastrula owes to an ances- 
tor more or less like the gastrula of Amphibia, but which has gained in the Teleost a 
distinctive character, owing to its apiiearance all round the lip of the blastopore at a 
time when the latter is verj" large. 
Significance of the germ ring tvith respeet to the amniotie gastrula.-— The fact that 
the ventral mesoderm, which in Amphibia is an important organ of the gastrula, is in 
Teleostei reduced to a rudimentary organ round the edge of the blastoderm, acquires 
a peculiar siguiflcaiice when the still more complicated gastrula of amniotes comes up 
for explanation. The fundamental features of this gastrula, it seemed, were satis- 
factorily explained by the Balfour-Eauber hypothesis, according to which the primitive 
streak plus the blastoderm edge represents the blastopore, the dorsal lip of which is 
indicated by the ueurentric canal. The manner in which these writers believed the 
lirimitive streak to have been phylogenetically formed, was thought to receive a con- 
firmation from the actual concrescence of the blastoderm edge, which takes place in 
the Selachian embryo behind the neurentric canal, During the last few years, how- 
ever, the tide has set against this hyjiothesis, and in the direction of a new one, the 
chief exponents of which are Kupffer, Cunningham, Oscar Hertwig, and Kabl. 
Kupff'er’s work on reptiles (25, 26), in the course of which he found that in this 
group there is no primitive streak, but in its place a definite invagination, led to the 
first step in the new direction. Kupffer came to regard the reptilian “ invagination ” 
(prostoma) and its homologue the Sauropsidan (and mammalian) primitive streak as 
alone representing the blastopore ; the edge of the blastoderm was looked on as totally 
independent of the blastopore and was explained in a very unique fashion. 
Cunningham (8), Oscar Hertwig (22), and Kabl (38) have all adopted this view of 
what constitutes the blastopore in the Amniotie gastrula. Hertwig says in his Lehr- 
buch (p. 104) : 
Als Urnmnd schlage ich vor nur diejeiiige Stelle des Keims zu bezeicliiien, an -welclier wirklicli 
wie bei der Gastrulabilduug des Ainphioxus und der Amphibien, eine Eiustiilpung von Zelleu statt- 
lindet, wodurcb die Furcbungsbdlile verdriingt wird. 
Such a process, according to Hertwig, does not occur round the edge of the blas- 
toderm, and is only found in the region of the primitive streak and the “prostoma” 
of reptiles. The edge of the blastoderm is hence not a part of the blastopore ; it is — 
eine Besonderbeit der meroblastiscben Eier, die nut der Entstelinng der partiellen Furcbuug auf 
das iunigste znsaminen liiiugt. 
