"722 Bashford Dean Memorial Volume 



had not been opened. Under these conditions development must have been almost 

 abnormally rapid. As a somewhat parallel case, I note that embryos and larvae of the 

 amphibian Cryptohranchus alleghemensts, kept during winter at moderate temperatures in 

 a basement, developed much more rapidly than embryos and larvae of the same species 

 left in their natural environment — in cavities under rocks in a stream often frozen over. 



Much of the evidence here presented is complicated by the fact that egg laying may 

 occur at any tmie of the year, though most often in spring. Moreover, it is obvious that 

 much variation in the rate of development is to be expected because of differences in 

 temperature, seasonal and otherwise. We have Dean's statement that eggs were col- 

 lected, in one instance, at a depth of 28 feet, and that Heterodontus japonicus is known to 

 inhabit depths varying from 3 to 20 fathoms (18 to 120 feet). At the maximum depth the 

 water is presumably much colder than it is near the surface, or in a laboratory aquarium. 

 Aeration of the eggs is another factor to be considered. An egg exposed to water cur- 

 rents, particularly an offshore current, is presumably better aerated than one shut off from 

 such currents. Lacking adequate data, it would be rash to attempt to estimate the 

 amount of variation in the rate of development due to environmental causes, but it must 

 be considerable. 



GENERAL MODE OF DEVELOPMENT 



This subtitle is inserted mainly to afford an opportunity to introduce at this point 

 Dean's evaluation of his results from the study of the embryology of Heterodontus japoyiv 

 cus, and his hopes for future accomplishments, in his own words taken from his brief and 

 very incomplete manuscript: 



Heterodontus, although separated from its nearest genera during long ages (at least since 

 the earliest Mesozoic), exhibits a plan of development not differing greatly from that de- 

 scribed among sharks of the present time. The egg is about the same relative size, its en- 

 velopes are similar, its early development follows the same course, its embryos have essentially 

 the same forms as Scyllium, Pristiurus or Squalus. It must not, however, be concluded that 

 its embryology is lacking in interest, for, as will be seen in the following pages, the differences 

 which occur in Cestraciont development are in clear accord with its more ancient Hneage, and 

 we will find that these differences will give us an interesting hght on the puzzling question 

 of to what degree development may in time come to be modified. It will be seen, for example, 

 that a Cestraciont still retains traces of an holoblastic cleavage, and that its blastoderm still 

 grows around the egg before the embryo is of large size, features which stand clearly in the 

 gap which has separated the plan of development of recent sharks from that which occurs in 

 ganoids and lungfishes, a plan which, on many grounds, must also have existed in primitive 

 sharks. But these considerations may best be examined in later pages of our work. 



In a somewhat similar vein, Haswell (1898) had previously written concerning 

 Heterodontus phillipi: ". . . the hope is not an unreasonably sanguine one that the em- 

 bryonic development of a type so ancient might exhibit some important primitive features. 

 With regard to the stages now described, however, any expectation of the kind cannot be 

 said to have been fulfilled ; and what impresses one most is the extraordinary persistence 



