260 



s. WATASE 



Thus, there is in the embryo as in the adult, a complete series of 

 gradations between the paired and the unpaired form of fins. The 

 anomalous condition observed in the anal and caudal fins of gold- 

 fishes is apparent from the very begining of their existence. 



Preanal Folds. The structure known as the preanal fold is 

 usually well-marked in gold-fish embryos. It often occurs in a 

 paired state extending from the anus up to the level of the spots 

 Avhere the ventral fins appear later (Fig. 33, p[a[f',) The two often 

 unite at their anterior ends into a median fold (Fig. 29, m.) 



The latter extends further anteriorly beyond the level of the 

 ventral fins. The preanal folds originate as ridges of ectodermal cells 

 like the caudal or anal fins. 



The existence of double preanal folds in the embryo is another 

 interesting feature of gold-fishes. Thus there exists a pair of con- 

 tinuous longitudinal folds running along the ventral side of the body 

 from the level of ventral fins down to the very end of the body, save 

 the single local interruption on account of the existence of the anus 

 (Figs. 24, 25, 26, 28). 



To show how such an embryo gold-fish differs from the embryo 

 of a normal teleost in this respect, I refer to Figs. 24 and 25. I have 

 connected the roots of the fins and preanal folds with dotted lines. 

 Fig. 25 shows the ventral aspect of a young gold-fish, half an inch 

 in length, in which the ventral fins have just begun to bud out and 

 the double preanal folds are in process of atrophying. The above- 

 mentioned dotted lines are furthest apart from ench other at the pectoral 

 region and slightly converge toward the ventral fin; behind the 

 latter, they suddenly approach each other, almost meeting in front of 

 the anus, behind which the two again gradually diverge. 



At the commencement of the caudal fin the divergence becomes 

 suddenly marked and finally they meet with each other. In a 



