450 Mr. E. E. Prince on the 



rim, whose growing (inner) margin is, at first, parallel to the 

 circumference of the disc, the latter by epibolic extension 

 thins out, and presents in optical section a crescentiform 

 outline. At one point, however, by a proliferation of epiblast 

 cells, a thickening is produced coincident with one of the 

 radii of the disc. 



Formation of the Embryo. 



The thickened portion of the rim, just mentioned, shows 

 from the first a central enlargement, indicating the future 

 head of the embryo, and an alar expansion upon each side 

 produces a broad scutiform outline. Tlie apex of this scutum 

 becomes the permanent cephalic extremity, and prominently 

 projects as a protruding carina upon the sub-blastodermic 

 matrix, whilst posteriorly the tapering trunk of the embryo is 

 gradually defined. The greater part of this embryonic 

 thickening is made up of epiblast cells, which constitute the 

 axial (neural) cord. This cord grows downward and divides 

 the undifferentiated " lower layer " cells into two lateral 

 cuneate masses, out of which the muscle-plates are built. The 

 dorsum of the embryo is superficially rounded and projecting, 

 showing no trace whatever of a longitudinal medullary groove, 

 and there is no ingrowtli of the corneous layer, such as Cal- 

 berla describes in St/ngnathus, the neurochord arising as a 

 solid rod in which for some time no neural canal develops. 

 It is interesting to note that Petromyzon precisely agrees 

 with the Teleosteans in this feature, and, in both, the medullary 

 canal arises as a fissure, which appears at first in the thick- 

 ened anterior portion and extends posteriorly, the process 

 being simply one of dehiscence, the central cells separating 

 to form a longitudinal vertical fissure *. The epidermic layer 

 now separates from the neurochord, and the hypoblast becomes 

 thickened along the ventral median line and presses upward 

 against the ridge of the neurochord, which is thus some- 

 what flattened on its under surface. The central rod of hypo- 

 blast, thus differentiated and detached, is the notochord, and 

 posteriorly it is insensibly lost in a caudal mass of indifferent 

 cells. At an early stage, before the notochord is completely 

 established, its cells are in close apposition to the meso- 

 blast cells upon each side, and the two masses can with 

 difficulty be distinguished from each other. This difficulty of 

 clearly distinguishing the cells of the different layers is one 

 common to all the early stages of Teleostean development. 



Meanwhile the blastoderm is proceeding epibolically to 



* Shipley, " Nervous System of Petromyzon,''' Cambr. Philos. Soc. 

 March 1886 j and Scott, Quart. Journ. J\Jicr. Sci. vol. xxi. p. 145. 



