450 MORGAN. [Vol. X. 



Teleost embryo. The blastoderm flattens, and its perimeter 

 thickens to form the germ-ring. From the very beginning the 

 germ-ring is thickened at one point, and this differentiation 

 becomes more marked while the anterior embryonic foundation 

 is formed. The elongation of the embryo behind this point is 

 due to the union of the two germ-bands that had before been 

 separated. The middle portion of the embryo is added in this 

 way to the head, and the posterior portion to the middle. " So 

 wird aus einem Ringtheil des Keimes ein Achsentheil des 

 Embryo. Die totale Embryonalanlage ist also das Ergebniss 

 eines Conjunctionsphanomens." The formation of the embryo 

 along one meridian is due to unequal growth of different por- 

 tions of the embryonic material. 



This interpretation of the normal development Rauber be- 

 lieves necessary to account for the various forms of abnormali- 

 ties described in his paper. This view of the concrescence of 

 the embryo is practically the earlier view of His. 



Rauber has figured quite a number of most interesting abnor- 

 malities, and has attempted to explain, on the concrescence the- 

 ory, many of the results as due to a "Conjunctionsphanomens." 



There are two main classes of these abnormal embryos. In 

 the first class we may place those embryos where, apparently, 

 the two sides of the embryo are laid down separately, and 

 ,at some distance from one another, united anteriorly by the 

 anterior end of the medullary plate. An inverted V-shaped 

 structure is produced with the two limbs of the V running out 

 into the germ-ring. Rauber attempts to explain this condition 

 as the result of an imperfect apposition of the two sides of the 

 embryo, due to a lack of nnioji of the rigJit and left sides of the 

 germ-ring. The yolk, however, is not exposed between the two 

 limbs of the V, as it should be if his hypothesis is collect. But 

 there is even a more serious objection to Rauber's interpretation. 



I have tried to show that the anterior end of the fish 

 embryo does not form by the apposition of the germ-ring, but 

 that it is laid down before the germ-ring has begun to grow 

 over the yolk, and that this portion of the embryo reaches from 

 the germ-ring, where it is most thickened, to the apex of the 

 earlier blastoderm, forming in situ. 



