ZOOLOaY AND BOTANY, MIOROSOOPY, ETC. 743 



condition of a functional monostielious eye, the deep ends of whose retinal 

 cells were directly continuous with the optic-nerve fibres ; in the earliest 

 stage of the present eye, before the appearance of the bacilli, the nerve- 

 fibres emerge from the outer and posterior border of the retinal infolding 

 immediately underneath the lentigen ; on the develoiiment of the bacilli 

 the fibres emerge further and further back from the surface of the head, 

 until at last the nerve is separated by a considerable interval from the 

 lentigenous cells. As the author points out, " this is exactly what might 

 have been expected if the eye had been developed phylogenetically by the 

 inversion of a layer of cells which were already in functional activity 

 before the process of inversion began, and the deep ends of which were 

 connected with the optic nerves. It is also consistent with the formation 

 at the deep ends of the retinal cells of secondary bacilli, which may be 

 regarded as the physical cause of a recession (ontogenetic) of the place 

 where the optic nerve emerges." It is possible that the primitive bacilli 

 do not in all cases completely atrophy ; such a hypothesis would, at any 

 rate, explain the problematic bodies which are found in the retina of 

 scorpions — the phaospheres of Laukester and Bonnier. 



The tapetum, to the presence of which it is possible to ascribe the 

 retention of the original bacilli, does not seem to owe its iridescent scales 

 to the cuticular secretions of the hypodermis. Mr. Mark thinks it more 

 likely that the tapetum is formed from cells which grow from the apex of 

 the original retinal involution into the cavity formed by that involution, 

 and that they take the form of an outfolding. There is reason to suppose 

 that the course of the optic nerve-fibres through the post-tapetal layers is 

 a secondary condition ; if, as is probable, post-nuclear eyes were developed 

 from functional monostielious eyes, the deep ends of whose retinal cells 

 were directly connected with the nerve-fibres, the fibres should retain their 

 connection with the deep ends of the cells, and should, even in advanced 

 stages, exhibit a course similar to that pursued by the nerve in " pre- 

 nuclear " eyes at an early stage ; but, instead of this, they traverse the 

 post-retinal layer. This, however, is only a modification, and not a 

 fundamental difference. 



a. Insecta. 



Directive Corpuscles in Eg-gs of Insects.* — Dr. F. Blochmann is of 

 opinion that a sufficiently large number of eggs of Insects have now been 

 examined to justify us in speaking definitely. In Blatta and in the 

 Aphides directive corpuscles are formed in exactly the same way as in 

 most other animals ; but in Musca there are certain modifications ; to put 

 this in another way, we find that the more primitive forms, the Orthoptei'a 

 and the Hemiptera have retained the primitive mode, while there are 

 changes in the more differentiated Diptera ; this may, perhaps, be corre- 

 lated with what we know as to the various secondary modifications which 

 obtain in the development of Musca ; at any rate, it is striking that these 

 cenogenetic processes should be observable in the very earliest stages of 

 development. 



In Blatta we can with absolute certainty, and in Musca with great 

 probability, assert that the point of exit of the directive corpuscle marks 

 the dorsal side of the future embryo, and herein the eggs of insects agree 

 with those of other animals. It is still more important to note that this 

 point of exit marks the animal pole, and may be taken as a fixed point in 

 the topographical relations of the germinal stripe. 



* Morphol. Jahrb., xii. (1887) pp. 544-74 (2 pis.). 



