MUSEUM OF COMPAKATIVE 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 basement 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 (PI. 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 (PI. III. figs. 14, 15, nl. 

 ms d.). Although mesodermic nuclei occur between the two retinas, and 

 also between the retina and the brain, tliey 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 (PI. III. fig. 15, PL II. 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 III.) 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 



