ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 9 



The third stage is marked by the complete absence of all traces of 

 the vertebral segments ; these, however, gradually disappear. The 

 intervertebral segments become still more completely dissociated, in a 

 manner for the right understanding of which figm-es are necessary. In 

 the human foetus after birth indications of these segments may yet be 

 seen. The " pulpy nucleus " of each segment is chiefly formed by 

 the perichordal tissue. 



A comparison of mammalian embryos with those of the chick 

 demonstrates modifications of detail, in accordance with the particular 

 forms eventually assumed by the articulating ends of the vertebral 

 bodies. 



Embryology of Selachians.* — Well-preserved embryos of Acan- 

 iJiias from Heligoland were examined by Dr. H. Rabl-Eiickhard, of the 

 Berlin Museum, with a view to determine (I.) the cephalic ending of 

 the chorda dorsalis, in its relations towards the hypophysis cerebri 

 and the so-called middle trabecula, and (II.) the origin of the pineal 

 gland. 



I. Since the publication of Balfour's monograph, Reichert has 

 described the embryo of an Acanthias whoso notochord he believed 

 could be traced through the cranial floor in front of and below the 

 hypophysis. Dr. Eabl-Riickhard has been fortunate enough to find an 

 embryo of the same stage of development as Reichert's, corresponding 

 to that which Balfour indicates by the letter K. The appearances 

 presented by dorso-ventral sections of the head in this and in older 

 embryos are described and figured. Other figures, chiefly showing 

 transverse sections of various embryos, are given. And there is a 

 dorso-ventral section of an embryo of Mustelus vulgaris. From a 

 comparison of several preparations Dr. Eabl-Eiickhard concludes 

 that — 



(1) At no period of its development has the embryo of Acanthias 

 a notochord with its apex projecting beyond that part of the base of 

 the skull which subsequently becomes the dorsum sellte ; 



(2) The hypophysis arises immediately in front of the apex of the 

 notochord in the basal portion of that deposit of connective tissue 

 which is termed the middle cranial trabecula ; 



(3) The summit of this rudiment (Eeichert's processus sellse 

 turcicae) does not pass into the later sella turcica, but becomes the 

 adventitia of the basilar artery. 



On the whole, the author confirms, against Eeichert, the previous 

 conclusions of W. Miiller, Balfour, and Parker. But though it is 

 certain that the notochord stops short of the hypophysis and lies 

 behind (not beneath) it, this is not irreconcilable with Eeichert's 

 other statement (by him injudiciously confounded with his first) that 

 the chorda of young sharks at a certain period of development reaches 

 to the frontal wall (Stirmoand). The cephalic flexure shows us that 

 such a state of things is quite possible. 



The minute structure of the notochord, as seen in sections, is 

 described with its axial string of large, clear, vesicular cells, forming 



* Morph. Jahrb. (Gegenbaur), vi. (1880) pp. 535-70 (2 pis.). 



