1814.] 97 [Hyatt. 



The ova of sponges, during segmentation, afford some very- 

 striking illustrations of the action of the law of concentration in 

 the more specialized forms of the Carneospongiae as contrasted 

 with the less specialized Calcispongiae. The embryos, as we have 

 stated above, become differentiated in the bicellular or tricellular 

 stage in the Carneospongiae, while in Calcispongiae the similar 

 differentiation at the poles of the embryo does not occur until the 

 seven- to eight-celled stages. The embryo of Calcispongiae is also 

 a placula until the same stage, while that of Carneospongiae is an 

 amphimorula in the three-celled stage. The differentiation of the 

 embryo into ectoblastic and endoblastic layers begins, therefore, 

 earlier in the more highly specialized Carneospongiae than in the 

 more generalized and direct descendants of the prototype, the 

 Calcispongiae. If we are correct in our mode of looking upon 

 such phenomena this indicates that the lower sponges sprang 

 from single layered or monoplaculate Protozoa and that this stage 

 became lost in Carneospongiae, and possibly in some specialized or 

 abnormal Calcispongiae, through the action of the law of concen- 

 tration in development. The following stages exhibit the action 

 of the same law. Thus in Sycandra the cells of the central cav- 

 ity, according to Schultze, lose the collars and cilia, becoming 

 smooth in aspect like the epithelium of the adult in the lining 

 membrane of the same cavity. Then follows at a later adoles- 

 cent stage the ampullinula which occurs in Carneospongiae at a 

 much earlier period in the young. In this transformation the 

 sycon appears, the mesoderm thickens, the ampullae are formed 

 as open bags radially arranged around the central cavity and 

 extending outwardly form large sacs opening externally through 

 the pores. The loss of the cilia and collars is thus proved to be 

 a secondary transformation and may be attributed to specializa- 



those of Lankester (Journ. Mici\ Sci., vol. xvn, p. 411) on " precocious segmentation " 

 are by implication similar, since he attributes the origin of characters to adaptation 

 and concentration of development to heredity. The idea of precocity is, however, 

 pathological and has, as we have tried to show, a special application to the concen- 

 trated development of diseased forms. We have discussed these questions more at 

 length in Genesis of Planorbis at Steinheim (Anniv. Mem., Bost. Soc. Nat. Hist., 1880; 

 also, Proc. Am. Ass. Adv. Sci., vol. xxix, 1880; and Foss. Ceph. Mus. Comp. Zool. 

 ibid., vol. xxxn, 1883; and Science, nos. 52-53, 1884.) Dr. A. S. Packard (Stand. 

 Nat. Hist. vol. I, p. liii, Bost. Cassino) has stated his opinion, which coincides very 

 nearly with the above, especially in the rejection of natural selection as a fundamental 

 cause of variation. 



PROCKXDIKC9 B. It. X. H. VOL. XXIII. 7 FEBRUARY, 18S5. 



