MUSEUM OF COMPAKATIVE ZOOLOGY. 89 



the large elongated nuclei of the nerve-end cells and the flattened nuclei 

 of the inner layer. In this stage (PI. X. fig. 69) the essential features of 

 the eye are established, and it is possible to affirm positively that the 

 anterior median eyes in Agelena ncevia belong to the type in which the 

 nuclei of the retinal cells are post-hadllar. 



The three remaining pairs of eyes originate somewhat later, but in sub- 

 stantially the same way as the pair just described ; a hypodermic thick- 

 ening, a backward directed infolding which inverts the thickened region 

 and carries in beneath it a thin layer of hypodermis, the closure of the 

 orifice of involution, and the detachment of the involuted mass from the 

 hypoderm. The lens is also produced from modified hypodermic cells 

 resembling, though shorter than, those forming the lens of the median 

 anterior pair. But the two layers of the infolded mass do not undergo 

 the same changes as do the corresponding layers in the pair of eyes pre- 

 viously described. In the first place, the two layers remain permanently 

 (up to my latest stage, ten days after hatching) separated by the devel- 

 opment of a (in hardened specimens) much folded chitinous layer, which 

 is probably homologous with the cuticular covering of the body, with 

 which in the earlier stages it appears to be continuous. Secondly, while 

 the retina is developed as in the anterior eyes, from the cells of tlie 

 inverted portion of the infolded region, the bacilli do not arise in the 

 ends of the cells which adjoin the vitreous body, but at the opposite or 

 posterior ends. They are, therefore, found in the immediate vicinity of 

 the chitinous substance. The nuclei, in the latest stages examined, still 

 continue to occupy the anterior portion of the layer. Whether they are 

 vdtiraately displaced to the margin of the retina, I am not at present able 

 to say. Clearly, however, the retina is developed out of the middle layer, 

 as in the previous case, but the nuclei of the retinal cells are pre-hacillar 

 in position. About the time of hatching nerve filaments grow out from 

 the brain, and thus connect the cerebral ganglia with the retinal portion 

 of the eye. 



3. The lungs arise as a pair of extensive invaginations at about the 

 same time as the proctodseum. In sagittal sections of early stages the 

 lungs appear as oblong plates of cells, the large oval nuclei of which are 

 arranged in parallel rows (PI. XI. fig. 73). The cells forming the ven- 

 tral wall of the floor over the lung sacks, liowever, are several layers 

 deep, and their nuclei are not arranged in parallel rows as the other 

 nuclei are. The nuclei of the parallel rows undergo a change of form, 

 becoming flattened on one side and very convex on the other. In each 

 single row the convex faces look in the same direction, but the rows are 



