October 9, 1896.] 



SCIENCE. 



5S5 



EMBRYOS OF THE SMOOTH DOGFISH (GALEUS 



CANIS). 



During the early part of September, while 

 working on some integumentary structures of 

 the selachians at the Marine Biological Labora- 

 tories, Woods Holl, Mass., I was fortunate 

 enough to secure two female dogfish which 

 had been kept during the summer in the U. 

 S. Fish Commission aquaria. Each fish meas- 

 ured 1.07 m. from the end of the snout to tip 

 of tail. On opening their body cavities I se- 

 cured three embryos from one and four from 

 the other, the smallest measuring 84 mm., the 

 largest 89 mm. The specimens had well-devel- 

 oped external gills, and were attached by long 

 spirally twisted umbilical cords to the yolk sac, 

 which was still filled with abundant yolk ma- 

 terial. One of the embryos has been drawn by 

 Mr. Hayashi, the laboratory artist, and serial 

 sections have been made of another. Fuller 

 description and figures will be given in my 

 paper on the Ampullae of Lorenzini of the Se- 

 lachian Fishes. 



The condition of the genital organs of the 

 females was interesting. In the first specimen 

 the three embryos were all contained in the 

 right uterus, the left being empty, although the 

 walls of the latter were thickened and highly 

 vascular. At the anterior end of the body 

 cavity the dorsal region of the right ovary was 

 distended with immature eggs. Five of the 

 eggs, varying in diameter from 8 mm. to 15 mm., 

 were supplied with yellow yolk, and were 

 doubtless to be the eggs for next year's young. 

 Other white spherical bodies, presumably very 

 immature eggs, in size from \ mm. to 5 mm., 

 were thickly imbedded in the stroma of the 

 ovary. Both uteri were developed equally in 

 the second specimen, each carrying two em- 

 bryos. 



These facts seem to throw some light on the 

 breeding habits of this fish and suggest a means 

 of securing the stages now much needed in the 

 study of the development of the nervous system. 



The smooth dogfish is very abundant at 

 Woods Holl throughout the spring and is sup- 

 plied to the laboratory in great quantities from 

 the fishtraps. This species is viviparous, and if 

 the adult females are dissected in May or early 

 June they are found to carry eggs in the earlier 



embryonic stages. During July the dogfish be- 

 gins to leave these shores, and in August it is 

 impossible to get any material. Where the fish 

 spends the winter is not known. The embryos 

 secured during the latter part of July average 

 10-20 mm., or at most 40 mm. in length. So 

 far as I know, sizes larger than this have not 

 been taken before along the south shore of Mas- 

 sachusetts. When the fish reappear in the 

 early spring the embryos have reached the 

 'pup' stage, 15-20 cm. in length, and are often 

 born while the fish are in the traps. I have 

 never seen the ' pups' in the uterus of the female 

 later than May. 



Hence it seems certain that the breeding 

 habits of Galeiis are as follows : Eggs which 

 have received their yolk in the ovary during 

 the previous year begin their development in 

 the uterus in late spring. The embryos are 

 carried in the body of the mother until the next 

 April or May, when the young ' pups' , are ex- 

 truded. 



It is probable, therefore, that embryos of any 

 required length may be obtained if the large 

 females are secured in April or May, confined 

 in as natural surroundings as possible, and 

 killed when the young have reached the stage 

 of development desired. 



James E. Peabody. 



Woods Holl, September 10, 1896. 



THE LAW OF RHYTHMIC MOVEMENT. 



It has long been known that in such rhythmic 

 movements as walking, running, etc., a certain 

 frequency in the repetition of. the movement is 

 most favorable to the accomplishment of the 

 most work. Thus, to go the greatest distance 

 in steady traveling day by day the horse or the 

 bicyclist must move his limbs with a certain fre- 

 quency; not too fast, otherwise fatigue cuts 

 short the journey, and not too slow, otherwise 

 the journey is made unnecessarily short. This 

 frequency is a particular one for each individual 

 and for each condition in which he is found. 

 Any deviation from this particular frequency 

 diminishes the final result. 



Some measurements that I have already made 

 on natural and unnatural rhythms have sugges- 

 ted a law governing the amount of deviation 

 from the natural rhythm and the resulting loss. 



