544 SUMMARY OF CURRENT RESEARCHES RELATING TO 



typical segmentation which, primitively, completely agrees with the 

 primary metamerism of the mesoblastic somites of the trunk. Of 

 these cephalic somites there are nine. We may, in fine, conclude that 

 the head of the Vertebrata ordinarily contains nine mesodermal 

 segments, which, like the segmental musculature, might become of 

 use to the specific cephalic organs ; but the earlier stages generally 

 disappear and the metameres are no longer clearly seen to be separate 

 segments. 



Ahlborn next addresses himself to the question of whether the 

 gill-arches are homodynamous with the ribs, and comes to the con- 

 clusion that the history of development clearly shows that the 

 metamerism of the gill-arches, which according to Gegenbaur's hypo- 

 thesis is an expression of the primary mesomerism of the skull, is 

 really nothing of the kind, but a segmentation which is caused by 

 the primary branchiomerism of the enteron, and completely inde- 

 pendent of the segmentation of the mesoderm. This conclusion is 

 found to be confirmed by what obtains in Petromyzon and the Anura ; 

 the whole answer may be summed up in saying that the ribs are, but 

 the gill-arches are not, segmental. 



The other problem proposed is : How far have the cerebral and 

 spinal nerves a segmental nature, of the kind supposed by Gegen- 

 baur ? No primary segmentation affects the nervous system. Neuro- 

 merism, therefore, is in the peripheral nervous system nothing 

 more than a secondary repetition of all the pre-existing metameric 

 phenomena in the body ; it is segmental, when the nerves are dis- 

 tributed to the segments of the body, but not in the branchiomerous 

 organs. 



If the view be just that the nine rudimentary cephalic segments 

 of the ancestor of the craniote Vertebrata were developed in just the 

 same way as the trunk-segments, and if, at the same time, the medulla 

 oblongata is a similar continuation of the spinal cord, we may con- 

 clude that there were primitively nine pairs of spinal nerves in the 

 hind-brain, of which the third, fourth, and sixth had only motor 

 roots. But at the same time the so-called spinal-like cerebral nerves 

 of the Craniota cannot, when we consider their morphological and 

 physiological significance and the secondary character of neuro- 

 merism, be any longer compared with the segmental pairs of spinal 

 nerves. 



Embryology of Alytes obstetricans.* — M. Heron-Eoyer. gives a 

 detailed account of the external modifications observed during the 

 embryonic development of Alytes obstetricans. For the purposes of 

 observation the eggs vs^ere placed on moist muslin between watch- 

 glasses, and kept exposed to light and to a warm temperature. The 

 egg has a large vitellus and a small cicatrix. Segmentation, which is 

 limited in extent, commences after 12 or 13 hours with a dorsal 

 streak with a broad, shallow blastopore. The vitellus is now spheri- 

 cal, but soon becomes oblong with the formation of the elongated 

 embryo. The embryo has paired ocular lobes anteriorly, and cor- 



* Bull. Soc. Zool. France, 1883, pp. 417-43(3 (1 pi.). 



