BIRDS. 357 



and presses off that membrane and a part of the yolk-sac from the 

 abdomen, and the chick leaves the egg 1 . 



There now remain for consideration the organs of animal life in 

 the class of birds. The mass of the spinal marrow is smaller 

 than that of the brain, whilst the reverse was observed in fishes 

 and reptiles. A narrow canal runs through its middle. Where 

 the nerves of the wings (plexus brachialis) arise, the spinal marrow, 

 which in the upper part of the neck is thin, becomes broader; a 

 still more remarkable swelling appears in the lumbar region, where 

 the nerves of the hind limbs arise. Here the posterior strands 

 separate laterally from each other, and the central canal expands 

 into a sac which is filled with a watery fluid. From this part 

 downwards the spinal marrow becomes constantly thinner, runs 

 through the tail, and finally terminates in a fine thread; a so-named 

 cauda equina is not present 2 . 



The brain is more fully developed than in the two preceding 

 classes. In the first place, its mass is larger; in general, at least, 

 it surpasses in relative magnitude that of most reptiles, and of all 

 fishes 3 . It entirely fills the cranial cavity. Again, the greater 



1 Amongst the numerous writings on the development of the chick we name only 

 M. MALPIGHII I>iss. epistolica de Formatione pulli in oro in his Opera omnia, Londini, 

 1687, fol. ii.; A. HALLER Deux Memoires sur la formation du cceur dans U poulet, 

 Lausanne, 1758; G. F. WOLFF Theoria generationis, Halae, 1759, 8vo (ed. sec. ibid. 

 1774) ; ejusd. Ueber die Bildung des Darmkanals im bebriiieten Hiihnchens, iibersetzt von. 

 J. F. MECKEL, Halle, 1812, 8vo; CHB. PANDER Dissertatio inaug. sistens historiam 

 metamorphoseos quam ovum incubatum prioribus quinque diebus subit. Wirceburgi, 1817, 

 8vo (and his Beitrage, with 14 beautiful plates by D'ALTON, already cited above) ; 

 K. E. VON BAER Ueber EntvncTcelungsgeschichte der Thiere, Kb'nigsberg, 1828, 4to, s. 

 i 140, and in BURDACH'S Physiologic, Bd. n. 2te AufL s. 335 446; lastly, the 

 excellent plates of M. P. ERDL, in the first part of his work, interrupted by his death, 

 Die Entmckelung des Menschen und des Hiihnchens, Leipzig, 1845, 4to. We could not 

 dwell upon the great difference of opinion of the latest writers with regard to the 

 layers of the germ and other questions not yet sufficiently cleared up, but we have 

 deviated only slightly from the description given in our first edition, for which the 

 investigations of VON BAER supplied the ground-work. 



2 Compare NICOLAI in REIL'S Archiv, XI. s. 156 219, with a figure of the entire 

 spinal marrow of a goose. 



3 The proportion is very various ; thus the ratio between the weight of the brain 

 and that of the whole body has been found in the sparrow as 1 : 25, in the chaffinch as 

 i : 22, in the goose as i : 300, HALLER Elem. Physiol. iv. pp. 9, 10 ; Cuv. Lee. d'Anat. 

 comp. ii. pp. 151, 152, 2e e"d. in. pp. 79, 80. Still smaller than in the goose is the 

 ratio in struthious birds ; in the Indian Casuary as i : 670 or even i : 1000, MECKEL 

 Archiv fur Anatomic und Physiol. vi. 1832, s. 352. 



