MUSEUM OF COMPARATIVE ZOOLOGY. 185 
tissue, and consequently cellular. His first argument is for the basement 
membrane of the unmodified hypodermis. He shows that this membrane 
passes off from the hypodermis and invests muscles. Arguing by analogy 
from Froriep’s conclusion that the sarcolemma of striate muscles in ver- 
tebrates is connective tissue, he maintains that the investment of the 
muscles in spiders and the continuous basement membrane are connective 
tissue. This argument of itself is scarcely convincing, for we do not 
know that the sarcolemma in vertebrates and arthropods has necessarily 
the same structure. Schimkewitsch used a second argument, which was 
more weighty, namely, that in the envelope of the eye (sclera) nuclei had 
been found. But the figures which illustrate this point are, as Mark 
(’87, p. 70) has stated, open to criticism. 
In the developing eye in Centrurus, the Saget membrane appears 
as a thin sharply defined structure bounding the deep ends of the hypo- 
dermal cells. It is continuous over the optic nerve, and unites with the 
membrane investing the brain (Pl. III. fig. 16, mb.). In the region of 
the preretinal membrane it is double, one layer limiting the lentigen, the 
other the retina (Pl. II. fig. 9, mb.). This confirms Mark’s theoretic con- 
clusion. In the earliest stages studied, mesodermic nuclei occur at inter- 
vals between the two membranes, except directly over the centre of the 
eye, where the two membranes are in contact (Pl. III. figs. 14, 15, nd. 
ms d.). Although mesodermic nuclei oceur between the two retinas, and 
also between the retina and the brain, they are-never found within the 
basement membrane of the eye region, as they are within the envelope of 
the brain. As the two layers of the preretinal membrane unite, the 
mesodermic cells, instead of being included between them, migrate toward 
the margins of the eye, and leave the preretinal membrane when com- 
pleted destitute of cellular elements. In the region of the sclera, how- 
ever, mesodermic nuclei, often very much flattened and always closely 
applied to the outside of the membrane, are distinguishable almost up to 
the adult state (Pl. III. fig. 15, Pl. Il. fig. 9, nl. ms d.). It is, therefore, 
nearly certain that some of the substance of this mesodermic covering 
enters into the formation of what is known as the “sclera.” In the 
adult sclera, however, no nuclei are visible, and besides it is by no means 
certain that these mesodermic cells form a continuous investment over 
the basement membrane, — perhaps nothing more than a network. 
The nuclei in the eye on the right of Schimkewitsch’s Figure 11 
(Pl. IIL.) are almost identical in appearance with those found in the 
young eyes of Centrurus, where they appear to occupy the middle of the 
membrane}; they are in reality outside it, as can be readily demonstrated — 
