No. 2.] FORMATION OF THE FISH EMBRYO. 447 



Henneguy (2) has given an excellent and brief account of 

 the history of the subject, and Hertwig (3) has given a longer 

 disquisition on the same topic. 



Henneguy has stated very clearly the standpoint of Kupffer, 

 Oellacher, His, Rauber, Cunningham, and Balfour, in respect 

 to the theory of concrescence, so that we need not here dwell 

 further on their views. There is one result, however, that 

 must be taken into account. His has shown that the volume 

 of the young fish after the closure of the blastopore, plus the 

 volume of the extra-embryonic region (ectodermal wall of the 

 yolk) and germ-ring, is about equal to the volume of fully 

 formed blastoderm. In other words, the protoplasmic sub- 

 stance of the later stages is no greater than that of the early 

 stages. The material of the germ-ring, therefore, must pass 

 either into the extra-embryonic region or into the embryo. 

 His' measurements go to show that the volume of the develop- 

 ing fish slowly increases, while that of the germ-ring decreases. 

 It follows, he thinks, that the fish grows at the expense of the 

 germ-ring. But it does not, I think, necessarily follow, as he 

 believes, that the whole of the elongation of the fish embryo is 

 directly due to addition to its posterior end from the germ-ring. 

 Henneguy, by a series of careful measurements of the em- 

 bryo of the trout, has shown very conclusively that other factors 

 than the germ-ring are at work during the elongation of the 

 embryo. At an early stage, when the medullary folds are 

 forming, and before the embryo has gotten half its full length, 

 Henneguy measured the distance from the posterior end of 

 the embryo to the anterior tip of the notochord. Let us call 

 this distance AC. Similarly, the distance from the posterior 

 end of the body to Kupffer's vesicle we may call AB. From 

 the anterior end of the notochord to the anterior end of the 

 head we may call CD. 



These measurements show tha\ the total length of the em- 

 bryo from this stage till the closure of the blastopore increases 

 by 1.90 mm. The distance separating Kupffer's vesicle from 

 the posterior end of the tail-knob {AB) only increases by 

 .055 mm. — an insignificant amount when compared with the 

 total elongation. Therefore, Kupffer's vesicle retains its posi- 



