THE EYES. 



G3 



invaginations of the cephalic lobes seem to show great resemblance in the two 

 groups, but it is not ascertained that they share in the Araneae, as in the 

 Scorpiones, in the formation of the eyes. On the contrary, it appears from the 

 published statements that the optic rudiments arise quite independently of 

 the semi-lunar depressions. We are inclined, nevertheless, to assume that 

 the part of the brain arising from infolding perhaps corresponds to the optic 

 ganglion, and that the folds which yield the eyes later enter into relation with 

 it. The middle part of the brain (protocerebrum, St. Remy, No. 12) might be 

 traced back to the thickening of the cephalic lobes, while the posterior part— 

 the rostro-mandibular ganglion — is probably to be regarded as a part of the 

 original chain of ganglia, as has already been shown. 



This conjecture as to a connection of the cephalic pits with the optic rudiments 

 is confirmed in a recent work by Kishinouye (No. 62). This work differs from 

 former publications on the subject of the development of the Araneid eye, in 

 actually assuming a connection of the eyes (anterior median eyes or principal 

 eyes) with the cephalic pits, and thus, in this respect, establishes greater agree- 

 ment with the Scorpiones. Kishinouye further found three segments in the 

 developing brain, such as Patten assumed for the rudiment of the brain of 

 Scorpio (p. 18). Unfortunately, the statements of both these authors are not 

 .sufficiently clear to enable us to understand their detailed accounts of the 

 development of the brain, and to bring them into agreement with the statements 

 of earlier observers. Kishinouye's accounts do not in any way agree with 

 Patten's, as may be seen by the relation of the optic rudiments to the different 

 sections of the brain. The former describes an invagination which is inde- 

 pendent of the cephalic pits : nothing certain is known, however, as to its 

 relation to the segments of the brain. Interesting and important conclusions 

 may be expected from thorough investigation of this subject. 



The statements of Schimkewitsch, that the cavities in the rudiments of the 

 brain are not to be derived from infoldings of the ectoderm, but owe their origin 

 to a later process of folding of the ganglionic mass after separation from the 

 ectoderm, contradicting the views of other authors, can meet with no approval. 



B. The Eyes. 



The eyes, in the Araneae, arise through a process of infolding 

 which closely resembles that described in connection with the median 

 eyes of Scorpio ; special modifications, however, occur in the Araneae. 

 Until lately a connection between the eye-folds and the depressions 

 on the cephalic lobes Avas not assumed ; it was thought, rather, that 

 the optic pits first arose when the cephalic pits had already closed.* 

 The formation of the former certainly occurs at a late period in the 

 development of the embryo, commencing when the latter is nearly 

 completed. There then appear, in the frontal region, several pairs of 

 (transverse 1 ?) slit-like depressions lying near each other, or one 

 behind another. In Pholcus there are said to be two (Claparede), 

 in Lycosa three pairs, but our knowledge as to these depressions and 



* [This view will be materially modified by the observations of Kishinouye, 

 mentioned below, should they prove correct. — Ed.] 



