166 SUMMARY OF CUKKENT RESEARCHES RELATING TO 



pronounced, and there is a short straight thorn-like spur, a brief 

 distance anterior to the hypopyle (the posterior distal exit). This spur 

 is worked by a powerful muscle, so that while it normally is kept in a 

 relaxed coudition against the side of the clasper, once the latter has 

 entered the oviduct of the female it can be erected, forming an 

 important organ of attachment. The extreme tip of the clasper, 

 posterior to the hypopyle, is also provided with a very small much- 

 curved claw, likewise movable, perhaps serving to rupture the hymen. 

 In the ocellated ray {Raia clrcularis) there are many differences. The 

 claspers are almost smooth ; the apopyle exists rather in name than in 

 reality, and is removed a considerable distance posterior to the cloaca ; 

 the clasper has a groove, not a tube ; the rhipidion is a fan ; the siphon 

 sac is filled with a remarkable gland somewhat like a date stone. Each 

 of the two lobes of the gland is compound and the components open by 

 papillae in a longitudinal groove of the gland. The siphon tube, which 

 forms the ultimate duct of the gland, does not debouch at the apopyle, 

 but is continued as a completely closed passage to a separate aperture 

 posterior to the hypopyle. In this case the muscular walls of the siphon 

 sac do not serve to propel spermatozoa ; they serve to inject the secretion 

 of the clasper gland into the female. The claspers have a minimum of 

 skeletal support, but they are very erectile, it may be to four times their 

 natural size. Such an erection is not needed in SeijlUum and Acanthias, 

 which are provided with dermal denticles and spurs, respectively ; Raia 

 relies on erection and on a relatively larger rhipidion for fixing purposes 

 during insemination. • J. A. T.. 



Developmental Rate and Structural Expression. — Charles E. 

 Stockard {Amer. Journ. Anat., 1921, 115-277, 6 pi. 32 figs.). A given 

 animal species passes through its embryonic stages at a specific rate of 

 development, probably dependent upon the rate of oxidation in the 

 protoplasm of the species. This developmental rate varies within 

 certain normal limits ; should variations in rate extend beyond these 

 limits, the developmental result frequently becomes modified and dis- 

 torted. The rate is not uniform throughout, but periods of rapid pro- 

 gress alternate with moments of slow rate or almost quiescence. But 

 development in most forms is continuous, though stopping completely 

 is universal among birds and, is known in several mammals. Dis- 

 continuity may be induced by temporarily lowering the surrounding 

 temperature and thereby rtducing the rate of oxidation, and by directly 

 cutting oft' the supply of oxygen. The effect varies with the moment, 

 for some moments are critical, especially when marked inequalities in 

 rate of cellular proliferation are taking place in different portions of the 

 blastoderm or embryo. Slowing does not have the same abnormal 

 results as stoppage, for the normal inequalities of rate are reduced in 

 the same proportion. Practically any deformity recorded in the litera- 

 ture, other than those due to germinal mutations, can be induced by 

 lowering the temperature. Duplicities are easily induced in the sea- 

 minnow, Fundulus heteroditus, and in the trout, by arrest or stoppage 

 before the process of blastopore formation has begun. Double chick 

 embryos are probably due to the eggs being laid (in a minority of cases) 

 l^efore the gastrulation has begun. The fall in temperature following 



