Embryonic History o/'Pteroplataja micrura. 7 



a Rliina-XikQ, form, and (2) through a Myliobatoid form, is 

 equally familiar. At any rate it is a suggestion that arises 

 quite naturally from an external view of the pectoral fins and 

 gill- slits of these embryos of PteroplaUva micrura. 



Professor G. B. Howes, in his most interesting paper " On 

 the Pectoral Fin-Skeleton of the Living Batoid Fishes " &c. 

 (P. Z. S. 1890, pp. 675-688), incidentally suggests an alliance 

 between Uldna and the Ceratopterine Myliobatoids ; and 

 HerrOtto Jaekel (SB. Ges. nat. Fr. Berlin, March 1890), in 

 a paper of great interest, for the knowledge of which I am 

 indebted to Professor Howes, has drawn attention to the 

 importance of the disposition of the gill-slits in relation to the 

 pectoral fins for tlie purposes of a natural (phylogenetic) 

 classification of the entire order, and has laid stress upon the 

 Batoid affinities of Rhina. 



6. Considerations as to the Origin of Aplacental Viviparity 

 among the Elasmohranchs. 



If it is premature to jump from these embryos back to their 

 supposed ancestral relatives, it is equally premature to attempt 

 from them alone to interpret the meaning of the aplacental 

 viviparity of the Batoid fishes as a whole. The subject, 

 however, is so very tempting that one cannot refrain from 

 recording certain suggestions that naturally arise out of an 

 examination of the yolk-sacs and umbilical cords of these 

 embryos of Pteroplatcea. 



The methods of reproduction among Elasmohranchs are 

 three, namely (1) oviparity, (2) viviparity with the forma- 

 tion of a placenta, and (3) viviparity without the formation 

 of a placenta. 



We know how the second naturally arises directly out of 

 the first ; the large egg is retained in the terminal portion of 

 the oviduct, and in the process of development, from early 

 common arrangements by which " nutriment from the yolk- 

 sac is brought to the embryo partly through the umbilical 

 canal and so into the intestine, and partly by means of blood- 

 vessels in the mesoblast of the yolk-sac " * and so into the 

 general circulation, we come at last in these viviparous forms 

 to later special arrangements by which, when the yolk is 

 finished, nutriment from the maternal blood-vessels in the 

 uterine mucous membrane is brought to the embryo by means 

 of the greatly developed foetal blood-vessels of a yolk-sac 

 which has now, after the disappearance of the yolk and the 



* Balfour, ' Comparative Embrj^ology,' 2nd edition, vol. ii. p. 64. 



