2G4 



SCIENCE. 



[X. S. Vol. XXI. No. 529. 



2. Rough, or 'Abyssinian,' coat is in 

 guinea-pigs dominant over normal or 

 smooth coat. 



3. The three coat characters mentioned 

 are independent of each other. Each may 

 exist either apart from or associated with 

 one or both of the others. 



4. Accordingly, cross-breeding involving 

 the three pairs of alternate characters re- 

 sults in the production ^^ltimately of eight 

 visibly different classes of individuals, but 

 of twenty-seven really different classes. 



5. The principle of gametic purity is 

 realized in a general way but not abso- 

 lutely, for cross-breeding induces variabil- 

 ity in the intensities of characters. 



6. The gametes formed by certain reces- 

 sive individuals are prepotent. This pre- 

 potency is hereditary. 



Tropical American Fresh-Water Fishes: 

 C. H. EiGENMANN, Indiana University. 



The Early Development of Chordatcs in 

 the Light of the Embryology of Ascid- 

 ians: E. G. Conklin, University of Penn- 

 sylvania. 



Owing to the high degree of differentia- 

 tion of the egg and early cleavage stages 

 of ascidians, the small number of cells pres- 

 ent during gastrulation and organogeny 

 and the known cell-lineage of the principal 

 organs of the larva, the ascidian egg is the 

 most favorable in the whole phylum of the 

 C.'hordata for an exact study of the early 

 development. Under these circumstances 

 it is worth while to compare the develop- 

 ment of ascidians with that of other chor- 

 dates, whatever may be thought of their 

 phylogenetic position in the phylum. 



In ascidians the animal (maturation) 

 pole of the egg finally comes to occupy a 

 position between the anterior and ventral 

 poles of the larva and the chief axis of the 

 egg is antero-ventral and postero-dorsal in 

 direction. In other chordates the axial 

 relations of the egg and larva are not cer- 

 tainly known, but there is considerable evi- 



dence that Amphioxus and the frog are 

 like the ascidians in this respect. 



In ascidians, the frog and possibly in 

 AmpJtioxus also the spermatozoon either 

 enters the egg at the posterior pole or 

 moves to this pole after its entrance ; in 

 the two foraier classes the copulation path 

 of the sperm within the egg lies in the 

 future median plane, though in the case of 

 the ascidians this plane is not determined 

 by the path of the sperm, but is already 

 established before fertilization. 



In ascidians the cleavage of the egg is 

 bilaterally symmetrical ; the same is the 

 case with several other classes of chordates ; 

 the resemblances between ascidians and 

 Amphioxus being especially close. Prob- 

 ably in all chordates with holoblastic 

 cleavage the third cleavage plane cuts off 

 four ectodermal cells at the animal pole ; 

 in ascidians the four cells at the vegetal 

 pole are endodermal, mesodermal and neu- 

 ral plate cells. 



In ascidians and amphibians the blastula 

 and gastrula are bilaterally symmetrical 

 and the closure of the blastopore takes 

 place chiefly by the overgrowth of the 

 dorsal lip ; probably the same is also true 

 of Amphioxus. 



Among ascidians the chorda and neural 

 plate arise from a crescent of chorda-neuro- 

 plasm which surrounds the anterior side of 

 the egg and gives rise to the dorsal lip of 

 the blastopore. In many respects this 

 crescent resembles the 'gray crescent' of 

 the frog's egg and it seems not unlikely 

 that here and in Amphioxus also the chorda 

 and neural plate arise as in ascidians. In 

 all of these classes the neural plate comes 

 from the outer layer of cells of the dorsal 

 lip, while the chorda comes from its inner 

 layer. In ascidians and the frog the an- 

 terior limit of the neural plate reaches 

 about one third of the way from the equator 

 to the animal pole; the same is probably 

 true of Amphioxus also. 



