GASTRULA OF SHARK AND CHIMERA. 



73 



We have emphasized these conditions of growth in Chimsera, since they serve, 

 I conclude, to explain the gastrulation of the shark, a process so puzzling that 

 Samassa (1895) has even gone so far as to deny its presence, sensu stricto, in this 

 group. According to the present interpretation the primitive shark had, like Chi- 

 maera, a blastopore which opened near but not at the rim of the blastoderm; in 

 this position it next became a rudimentary organ, since, apparently, the conditions 

 governing the increase of cells in the archenteron suffered a change inasmuch as 

 they came to receive their nutriment directly front the neighboring germinal wall 

 instead of indirectly, i. e., through a process of continued invagination at the 

 blastopore. Accordingly, in the development of modern sharks the blastopore 



Fig. 67. Diagrams comparing gastrulae of Chimaera and Selachian. A and B, Earlier and later stages in gastrula of Chimeera 

 colliei. C and 1~>, Earlier and later gastrulae of shark (mainly after Riickert). 



O, Archenteron. dl, Dorsal lip of blastopore. sc, Segmentation cavity, vl. Ventral lip of blastopore. 



fails to appear within the blastodermal disc, since here it has long been functionless. 

 But obviously the blastopore would again become important in the economy of 

 gastrulation, if nutritive material were brought into its neighborhood by any process 

 in the growth of the blastoderm or in the encroachment of the germinal wall. 

 Thus we may infer that it would again become a functional organ when its position 

 was transferred to the rim of the blastoderm. In this position it still occurs 

 exceptionally, as C. K. Hoffman has shown in Acanthias, * or it may indeed reopen 

 deeper under the rim of the blastoderm, as the majority of investigators maintain. 



*In a letter, which I am permitted to quote (July, 1903), from Professor Hoffman, the comparison is accepted as 

 follows: "In Chimaera the blastopore is located near and in Acanthias at the rim of the early blastoderm. For the 

 rest the archenteron and the open blastopore of Acanthias agree entirely with those of Chimgera. Acanthias forms 

 the bridge (in this regard) between Chimaera and other sharks and furnishes us the key to the problem of gastrulation 

 of the other sharks." 



