350 



the observations on Necturus we should be led to the direct inference 

 that the process of concrescence has evolved pari passu with the 

 relative increase in yolk material and is most pronounced in those forms 

 possessing the greatest quantity of such material. 



If we briefly trace the inception and rise of the "Theory of 

 Concrescence" we shall find that it took its origin from the phenomena 

 observed in forms possessing a relatively great quantity of yolk ; namely, 

 in the fishes. 



K. E. VON Baer was the first, so far as I am aware, to call 

 attention to the marginal thickening of the blastodisc of the fish and 

 to suggest that this thickening represents the anläge of the future 

 embryo. Lereboullet some thirty years later observed a number of 

 abnormal fish embryos, in which the head and tail were perfectly normal, 

 showing the usual bilateral symmetry, the body, however, being divided 

 in the median sagittal plane; the halves lying widely apart. In these 

 separated halves, the half organs were differentiated. These facts led 

 the author to believe that the halves of the marginal thickening (or 

 more properly the germ-ring) represent the halves of the future embryo. 

 His some ten years later made a series of careful measurements of the 

 developing fish embryo as compared with the decreasing extent of the 

 germ-ring. The results indicated that the normal embryo is formed 

 by the growing together of the halves of the germ-ring. Later studies 

 on the Selachian and Chick led the author to consider the mode of 

 embryo formation similar to that previously observed in the Teleost. 

 As a result of these studies the view of von Baer and Lereboullt 

 was adopted, elaborated and stated as the "Theory of Concrescence". 

 The observations of Rauber on Chick and Teleost, those of Semper 

 and Whitman on Annelids, together with those by RiJckert on 

 Torpedo, Ryder on Elacate, Locy on Squalus gave the theory staunch 

 support. 



A theory which so satisfactorily explained the tectonics of embryo 

 formation in the Fishes, Reptiles and Birds, might be expected to be 

 of wider application and was extended to the Amphibia upon the most 

 insufficient data. The only evidence which the older writers could find 

 to support the theory as applied to the Amphibia was the statement 

 made by Kowalewsky that the dorsal groove in the Frog, Amphioxus 

 and Ascidians represents the continuation of the gastral invagination. 

 Later observations by Roux and Hertwig on pathological embryos 

 led them to regard concrescence as the mode of embryo formation in 

 the Amphibia. In an earlier paper i) I have reviewed the evidence 



1) 1. c. 



