The Ernhryology of Heterodontus japonicus 



721 



Dean states that at first sight his list of embryos of various sizes seems to yield reason- 

 ably complete e\-idence that the entire term is twelve months or thereabouts. On the 

 other hand, he realises that a weak spot in the e\adence lies in the fact that the series of 

 later stages is not complete. He is not sure that stages such as those assigned to August- 

 September, October and November-December are the dominant ones for the months 

 that have been suggested. Specimens in these stages are not at all common, and the 

 range of size has become so varied that one cannot tell whether a stage such as the one 

 represented in Text-fagure 45b is really the sequent of the one portrayed in Text-figure 

 45a, or is much older (i.e., from an egg which -^-as deposited, say, in September of the 

 previous year). Development is probably much slower during the winter months. The 

 age of the embryo represented in Figure 82, plate \'^II, which is hardly less than seven or 

 eight months, might be 14 to 20 months. And at hatching the embr\'o, which can hardly 

 be less than 12 months old, is possibly aged 20 to 24 months. 



The only direct observations on the growth rate are not in favor of the vievp that the 

 incubation period greatly exceeds one year. A late embryo in its opened capsule was 

 placed in an aquarium on August 10. It measured 45 mm. in total length. On October 5, 

 that is, v,-ithin a Httle less than two months, it had attained a length of 110 millimeters. 

 This grov^th is so extraordinarily rapid for a shark that if the same rate were continued, 

 estimating roughly an increase in length of 30 mm. each month, the young fish would have 

 hatched by December or January- (the young at hatching measure about 180 mm.). This 

 would make the entire period of incubation, assuming that the egg was spa"wned in April, 

 from nine to ten months. But the experiment was probablv not conducted under strictly 

 natural conditions. The temperature of the aquarium must have been considerably 

 higher than that of the sea bottom: and if the egg, after opening of the capsule, was left 

 uncovered the embrj'o may have been better aerated than it would have been if the capsule 



Text-figure 45. 



Outlines representing stages in the development of Heterodontus japonicus. A, stage collected mainly diking 



August and September; B, stage presumably most abundant during the month of October. 



From drawings left by Bashfocd Dean. 



