538 



Bashford Dean 'Memorial Volume 



DURATION OF GESTATION 



The duration ot the period of gestation in the frilled shark is not known, and, because 

 of the habitat of the breeding fish and of the absence of any definite breeding season, it 

 cannot be ascertained by direct observation. However, it will be of interest to set forth 

 some facts that indicate that the period is protracted. The same factors of constant low 

 temperature, great pressure, and restricted oxygen supply, that lead to an extension of the 

 breeding season to cover the calendar year, would also be conducive to slow development 

 of the embryos and a lengthened period of gestation. 



Text-figure 4 

 Egg shell (measuring 128 mm.), egg (100 x 65 mm.), and embryo (43 mm.) of Chlamydo- 



selachus, reproduced in natural siz;e. 

 After Nishikawa, 1898, Fig. 1, pi. IV. 



An idea of the duration of this period may be gained by studying a series of growing 

 embryos and noting the relative diminution of their yolk sacs. But first one must endeavor 

 to establish the normal si2;e of the yolk mass at, or shortly following fertilization. The 

 matter of the varying sizes of the eggs of Chlamydoselachus will be taken up later. Here 

 ^ve are interested in the size of the eggs in blastula or gastrula stages or in early stages 

 of embryonic development. Only two investigators have studied such eggs. The first of 

 these, Nishikawa (1898), states that the eggs (in early stages of development, probably 

 segmentation), range from 65 to 75 mm. in short and from 102 to 124 mm. in long diameter. 

 He writes of other eggs ranging from 110 to 120 mm. in long diameter. These measure- 

 ments are probably made over the egg shell. Thus Text-figure 4 (his Fig. 1, tab. IV) 

 is 128 mm. long in a straight line over the horns of the shell, whereas the egg itself 

 measures 100 x 65 mm., and the embryo 43 mm. in length. The egg shown in Text-figure 4 

 is in natural size. 



