378 



SCIENCE. 



[N. S. Vol. XXI. No. 532. 



writer has nothing to add. As described, 

 a papiUa is formed, grows downward from 

 the gill axis, and is refiexed on itself, giv- 

 ing rise to the familiar U-shaped filament. 



Later filaments follow a very different 

 scheme, there being no such bending of an 

 originally simple filament. At the pos- 

 terior end of the curiously curved gill axis 

 a series of thin transverse ridges are de- 

 veloped. At first the edge of each ridge 

 is entire; but growth is early checked in 

 the center, so that the ridge is divided into 

 two flat, rounded lobes, corresponding re- 

 spectively to a. filament of the outer and 

 one of the inner gill plate. As each lobe 

 elongates it becomes perforated at its prox- 

 imal end, thus being resolved into the two 

 branches of a U-shaped filament, identical 

 in form with those first developed. 



An interesting parallel is seen in the 

 development of the interlamellar connec- 

 tions. The interlamellar connection, in its 

 finished form, is a simple bar, containing a 

 blood channel, and connecting the two 

 branches of one filament. In an early 

 stage of development the two branches of 

 the filament are connected by a continuous 

 plate of tissue, extending from the bend of 

 the filament upward for a short distance. 

 This stage is closely comparable with the 

 adult condition in Modiola and Area. 

 Later a perforation appears in this plate, 

 and the portion isolated above the perfora- 

 tion is transformed into the characteristic 

 bar-like connection. The results derived 

 from the study of isolated filaments have 

 been confirmed by the study of sections. 



The Effect of a Freezing Temperature on 

 the Development of the Frog's Egg: 

 T. H. Morgan, Columbia Univei-sity. 

 No abstract. 



Latent Characters and Reversion: W. E. 



Casti.e, Harvard University. 



1. The coat of the wild guinea-pig con- 

 tains at least two pigments, black and 



yellow, on the same hairs. In certain 

 varieties of domesticated guinea-pigs occur 

 (a) black only, (&) yellow only or (c) 

 neither pigment (albino variety). 



2. These color types obey Mendel's law 

 in heredity. In the order named (wild, 

 black, yellow, albino), each type dominates 

 all which follow it and is recessive to all 

 which precede. 



3. A recessive character disappears when 

 brought into the same zygote with the 

 corresponding dominant, but reappears dis- 

 tinct in half the gametes formed by the 

 hybrid zj'gote. Hence recessive characters 

 may exist unseen in individuals apparently 

 dominant, but are bound to reappear if 

 hybrid dominants are bred inter se. 



4. Dominant characters also may exist 

 unseen in recessive indi\dduals, but the 

 conditions of their reappearance are quite 

 different. Such unseen (not recessive) 

 characters may be called latent. Mating 

 of recessives which contain latent dominant 

 characters ordinarily produces only reces- 

 sive individuals. Cross breeding with the 

 dominant type is usually necessary to bring 

 latent characters into activity, though in 

 some eases where the latency was partial 

 only, cross-breeding of two different re- 

 cessive stocks has accomplished this result. 



5. Albino guinea-pigs, mice and rabbits, 

 transmit latent specific pigment characters 

 (as black or yellow). This can be demon- 

 strated by cross-breeding. Smooth-coated 

 guinea-pigs may contain, in a condition of 

 almost if not complete latency, the domi- 

 nant rough or resetted coat. 



6. Reversion (or atavism) is a name 

 which has been given to the reappearance 

 in a race of lost ancestral characters. The 

 matter has always been more or less mys- 

 terious, but would seem to consist in large 

 part, if not exclusively, in the becoming 

 active of characters which are latent. For 

 this process cross-breeding seems normally 

 to be essential, its function being either to 



