1892.] Embryology. 967 
medullary plate folded in and the lateral eyes were for a time of little 
use; the lateral eyes are actually present, as the author hopes to show, 
when the medullary groove first appears. 
Polyspermy in Vertebrates.—Dr. J. Rückert’ has advanced a 
most interesting explanation of the origin of the yolk-nuclei, para- 
blast-nuclei or merocyte nuclei of meroblastic vertebrate ova. Finding 
these nuclei in eggs of elasmobranchs during, or even before, the union 
of the ¢ and 9 pronuclei he was struck by their apparent identity 
with the male pronucleus. Later he found many sperms present 
before these yolk nuclei appeared, and also saw transition stages 
between the two. That this apparent origin of yolk nuclei from 
sperms may have been exceptional, abnormal, in the few cases 
observed becomes less probable when the very similar discoveries of 
Oppel in reptiles are considered. 
Oppel* observed numerous secondary sperm-nuclei in the eggs of 
Anguis fragilis even before the union of the primary male pronucleus 
with the female pronucleus, and found them common in eggs of 
Lacerta viridis and Tropidonotus natrix also, at the time of union of 
these chief nuclei. These secondary sperm-nuclei often lie beneath 
funnel-shaped depressions of the surface of the blastoderm ; they form 
no connection with the female pronucleus, yet undergo division, but 
soon degenerate and take no direct part in the formation of the 
embryo. Their significance remains, to Oppel, an open question. 
Polyspermy has been noticed in reptiles also by Todaro, in the 
trout by H. Blane, in petromyzon and in batrachians by von Kupffer 
and in insects by Henking and by Blochmann. 
These observations upon the wide occurrence of polyspermy, how- 
ever much they may favor the idea of the normal occurrence of poly- 
spermy in elasmobranchs, offered nv clue as to the fate of the supernu- 
merary sperms. 
To support his thesis that these sperms become the yolk nuclei, the 
author makes use of the following rather unsatisfactory evidence : 
Having shown that the merocyte nuclei cannot have arisen from the 
female pronucleus or from the segmentation nucleus, the question as 
to their origin narrows itself down to some form of external accession, 
free cell formation being excluded on general grounds. Of such 
external origin the possibility of inwandering maternal cells cannot 
be altogether denied, yet that many, possibly all, the yolk nuclei 
SAnatom. Anzeiger, vii. May, 1892. 
*Anatom. Anzeige?, vi, 1891. Also Archiv. f. Mik. Anat., xxxix, 1892. 
68 
