DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 731 



granular protoplasm with many nuclei, and cells apparently forming around them." 

 Anteriorly, the hypoblast still preserves its flattened character (hy, PL IV. fig. 5a), 

 while in the otocystic region it seems to merge in the neurochordal cells, ne, unless the 

 undefined cells in the middle line be a thin stratum of mesoblast, in course of formation, 

 and destined partially to constitute the nuchal and cephalic mesoblast (PL IV. fig. 4). 

 A similar indefinite axial tract occurs in the chick (No. 19, p. 184). Further forward the 

 hypoblast is once more fairty defined (PL IV. fig. 2), and at the tip of the snout, as before 

 mentioned, it may often be distinctly seen to overlap the epiblast as a thin veil (figs. 4-6 

 and 19, PL III.). 



The Blastopore. — The blastopore (Dotterloch — trou vitellaire) may be said to exist 

 from the moment that epiboly begins. It coincides with the margin of the germ, and 

 forms in fact the border of the saucer-like blastoderm at the conclusion of cleavage. 

 Later, however, it is more distinctly recognisable as a kind of spacious mouth, from which 

 the ball of yolk is seen projecting. Small granules often occur plentifully at the margin 

 of the rim, and are imbedded in the periblastic ring (PL III. fig. 16). The continued 

 extension of the germ over the yolk produces certain changes, notably in its diameter, 

 which are easily observed. 



Contrary to Oellacher's view, the rim seems to progress at an equal pace at all 

 points, and it thus increases in diameter until the process of enclosure is half accom- 

 plished; but after the equator is passed, the aperture necessarily diminishes, and finally 

 presents a fairly circular form. Oellacher regards the caudal end of the embryo as 

 a fixed point, so that the parts of the rim further away from this point advance at an 

 increased rate — progress being, in fact, rapid in proportion to their remoteness (No. 114, 

 p. 4). This assumption, however, is very questionable, the snout of the embryo being 

 apparently the fixed point, while increase in length takes place in the caudal region. 

 No part of the rim can be shown to be stationary, for the embryo lengthens as epiboly 

 proceeds, and no part presents more signs of active growth and development than the 

 posterior extremity, as already indicated. 



The lip of the invaginated rim, for which the name blastopore is on every ground 

 justifiable, attains its maximum size when the equator is reached, and after that stage it 

 continues to diminish until finally it closes. Often it assumes an oval form (bp, PL III. 

 figs. 7, 23 ; and PL XXVIII. fig. 5), doubtless due to the plastic nature of the yolk; but 

 usually an almost perfectly circular outline is preserved. In some cases the blastopore 

 has the rude outline of a flask, the narrow portion forming a bay, which coincides with 

 the caudal end of the embryo, and this has suggested the theory of concrescence in these 

 forms. In most cases no such terminal bay is seen, the embryo in fact projecting more 

 or less prominently, and breaking the circular outline of the blastopore in a manner 

 exactly the reverse of that just mentioned. In the later stages of development in 

 ovo the concrescence theory is not clearly borne out, e.g., by the view of the gurnard 

 (PL XIV. fig. 7), in which the rim forms a backward loop at the tail. This concrescence, 

 however, may occur without a visible bay or angle directed forward, as indicated by 



