26 
mena of Vertebrate development which have hitherto prevented the 
enunciation of “the law of development as an alternation of generations”. 
Larvae are so commonly encountered among the Invertebrata, that the 
wonder is that no-one has enquired why they are so rare in any guise 
in the Vertebrata. 
In the latter division of the animal kingdom it becomes neces- 
sary to approach the problem previously stated as to the fate of the 
larva when uterine development is initiated. 
It may firstly be noted that larval forms, equipped with many 
adaptational larval organs, are to be encountered in cases with com- 
plete segmentation and but little food-yolk, e.g. Marsipobranchii, 
Ganoidei, and Amphibia, while a blastoderm on a yolk-sac is 
characteristic of Elasmobranchii, Teleostei, and Sauropsi- 
da, in which a larva, according to the common acceptation, would 
not be very obvious. 
In all these cases, however, larval organs can be proved to exist, 
and, most important of all, there is a well-marked larval nervous 
system, which, while not certainly known to persist in any adult form, 
has been proved to degenerate during the ontogeny of all the ovi- 
parous Ichthyopsida as yet studied. 
This apparatus is certainly neither a part of the adult nervous 
system nor homologous with the latter‘). 
Among other larval structures referred to when reading the paper 
the curious degenerating cells on the blastoderm of Pristiurus, to 
which van WISHE once drew my attention, and the knob?) on the 
blastoderm of Torpedo, as shown in ZirGuEr’s beautiful models of 
the embryos of this form, and as described by H. E. and F. ZIEGLER 
in the Archiv f. mikrosk. Anat., Bd. 39, p. 85, deserve mention. 
The yolk-sac viewed as part of the larva would require detailed 
and extended consideration. 
It is gradually broken down by some ferment action on the part 
of the so-called “merocytes”*), which may possibly represent de- 
generating cells of the larva. Only towards the close of life in the 
1) J. Brann, The trausient ganglion cells and their nerves in 
Raja batis. Anat. Anz, 1892, p. 191—206, and also: The early 
development of Lepidosteus osseus. Proc. Roy. Soc. Lond., Vol. 46, 
1889, p. 115—118. 
2) Possibly the remains of the ”umbrellar“ portion of the larva. 
3) It is to H. E. Zreerer that we owe most of our knowledge of 
the way in which these “merocytes” in their own degeneration and 
death, cause the elements of the yolk to become fit for absorption and 
assimilation, 
