102 ADAM SEDGWICK. 



selves continue to break up, but a kind of core remains 

 which constitutes the foundation of the future nerve and 

 ganglion. The Gasserian ganglion, the ophthalmicus pro- 

 fundus, the mandibular branch of the fifth, and the ciliary 

 ganglion thus gradually emerge from the remains of the nerve- 

 crest — are, so to speak, crystallised out of it. At first they 

 have the form of dense cords of nuclei ; but they soon acquire 

 some of the non-staining fibrous substance, which makes its 

 appearance as a rule in their central portions, so that for a 

 time sections of these nerves exactly resemble in appearance 

 sections of the nerves of Invertebrata, e. g. Peripatus, 

 Chiton, &c. This description holds for an embryo of 35 mm., 

 beyond which stage I have no observations. The nuclei which 

 have peeled ofi', leaving the nerve-trunk below, give rise to the 

 muscular and connective tissues of the parts concerned, the 

 reticulum of which is freely continuous with that of the nascent 

 nerve, especially at the free end of the latter. It thus be- 

 comes apparent that these tissues — nervous, muscular, 

 connective, and vascular — are all developed in continuity.^ 



While the Gasserian ganglion, the mandibular branch of the 

 fifth, the ophthalmicus profundus, and the ciliary ganglion all 

 crystallise out of the nerve crest ; the third nerve does not do 

 so. It arises as a difl^erentiation of the reticulum formed by 

 the breaking up of the nerve crest, and it first makes its 

 appearance as a forward projection of nuclei from the ciliary 

 ganglion. This, by a gradual difi^erentiation of the reticulum, 

 extends itself until it reaches the base of the mid-brain, with 

 which it becomes continuous by means of an increase in the 

 pale fibrous strands which pass between the medullary wall 

 and the reticulum. The third nerve is at first a cord of nuclei 

 and rather dense pale substance. The third nerve,^there- 



' The continuity of tlie embryonic tissue which will give rise to the 

 nervous and muscular tissues is well seen in the embryo of Peripatus 

 capensis, and I have already hinted at this fact in my Monograph on the 

 development of that species at pp. 131 and 133, and figured the tissue as 

 nerve muse, pi. x, fig. 5. 



' It will be evident, if my observations are correct, that I have found an 

 earlier stage of the third nerve than Dohrn describes in his sixteenth study. In 



