ZOOBOGY AND BOTANY, MICROSCOPY, ETC. 
603 
Evolution of Muscles.* — Prof. M. Nussbaum calls attention to tlie 
specific peculiarities in the musculature of Amphibians. It is not 
possible, he says, to understand these by anatomical studies of the 
adults alone. Still less does Palaeontology help. The variations have 
been embryonic and larval, and must be studied in these stages. If we 
understand the author aright (his paper is somewhat too condensed), his 
position is that the differences observed in the adult musculature can 
only be explained by working upwards from the variations occurring in 
the young forms, and not vice versa. 
Yolk-Sac and Merocytes in Scyllium and Lepidosteus.j — Dr. J. 
Beard calls attention to the lack of precise information as to the history 
of the yolk and the merocytes. At the “ critical stage ” in Scyllium (i.e. 
when the embryo, about 32 mm. in length, begins definitely to make for 
the adult form, suppressing its larval foundation), the genital ducts are 
practically established, the sex is defined both internally and externally, 
the retina is becoming pigmented, the first rudiments of scales are 
present, and for the first time there is a beginning of an internal yolk- 
sac containing yolk ; and then, or very soon after, yolk is found in the 
gut-cavity. 
During the earlier part of the development of Scyllium the yolk is 
absorbed solely by the blood-vessels of the sac, after being prepared 
for absorption by the yolk-nuclei or merocytes. These aggregate near 
to or along the blood-vessels of the sac. Mainly, if not entirely, they 
are products of segmentation, and may be regarded as really cells of the 
hypoblast. They appear to increase only by direct division until the 
critical period, after which they seem to become smaller and fewer. 
The yolk-sac circulation, as a means of nutrition of the embryo, becomes 
less and less important ; a new mode of nutrition is initiated. An 
internal yolk-sac of one thin layer of hypoblastic cells is formed, and 
the unmodified yolk is gradually drawn into this sac, and thence into 
the gut where it is acted on by digestive juices. The merocytes, still 
remaining in the external yolk-sac, degenerate. The internal hypo- 
blastic lining of the sac is broken up in situ and absorbed, and the 
entire sac — now empty — degenerates. 
In Lepidosteus also there is a marked critical period. Beard main- 
tains the correctness of his previous observations, as well as those of 
Balfour and Parker, as to the segmentation. This is said to show signs 
of being holoblastic (eight furrows being traceable to the centre of the 
lower pole), though Bashford Dean holds that it is thoroughly mero- 
blastic. The difference of opinion is interpreted by the author as duo 
to diverse modes of preparation. 
By the time the embryo has attained a length of 8 mm., and the 
yolk-plates in its cells have mostly been used up, the merocytes form 
a fairly even layer over and around the yolk. For a considerable 
distance they fill in the floor of the ventral wall of the alimentary canal ; 
thus the larval gut has, as its ventral wall, the merocytic layer on the 
top of the yolk-sac. At the critical period, when the embryo is about 
9 mm. in length, the embryonic gut is closed in except at one point, 
where there is a passage or yolk-bay leading to the yolk-sac. As in 
* Verb. Anat. Ges. X. Anat. Auzeig. Erg.-Heft, xii. (189G) pp. 61-7. 
t Anat. Anzeig., xii. (1896) pp. 334-47. 
