190 Memoir Sears Foundation for Marine Research 



been due to tank conditions, but it is interesting that it compares almost exactly with 

 the results obtained by Clark^ with R. brachyura kept in aquaria at Plymouth. These 

 data suggest the possibility that in nature not more than 10—25 % of the eggs laid 

 are fertile or develop beyond an early stage. In this connection, exceptions to the rule 

 that each egg case contains only a single egg or embryo in this species are rare. 

 From the many specimens we have examined we have found two eggs in a single egg 

 case twice, and four eggs in one case once; all three anomalies were detected in pregnant 

 fish, and in each instance the matching egg case was empty. 



Hatching of eggs laid in the May— July period and maintained under aquarium 

 conditions took place from November 29th, 1950 to January ist, 1951, that is, five to 

 six months after they were laid. This is probably the shortest time in which hatching 

 takes place in nature, since the water temperatures in the aquaria were somewhat 

 above those of the normal environment over the entire period and since eggs laid in 

 June and July would be subject to optimum temperatures for the fastest development 

 in these waters. Those laid in the November— January period probably take consider- 

 ably longer to reach hatching. It seems probable that the eggs of R. erinacea take a 

 minimum of six months and up to nine months or more after laying to arrive at the 

 hatching stage in nature. 



At hatching, Skates from Long Island Sound waters are 9.5-10.0 cm in total 

 length and the yolk in the abdomen is still apparent. As this time the tip of the tail 

 has a fine, distinctively pigmented, whip-lash elongation which disappears after a few 

 days; thus the distance from the base of the second dorsal to the end of the tail is 

 1. 4-1. 5 cm at hatching and only 0.925-0.950 cm several weeks later when this fil- 

 amentous piece has sloughed off. It seems quite possible that this 5 mm elongation is 

 directly concerned with hatching; in short, it may be comparable in function to the 

 egg-tooth in birds and similar devices in some Amphibia. Within the egg case before 

 hatching the Skate is oriented so that its head points toward the end of the case pos- 

 sessing the long straight tendrils. The transverse margin of the case at this end is 

 straight and sometimes frayed, and the two broadly convex surfaces of the case meet 

 approximately 5 mm from the margin to form a flat area at that end. It is via the separ- 

 ation of the two surfaces of the egg case in this area that the Skate escapes, and after 

 hatching this marginal area resembles an envelope which has been opened by a sharp 

 paper cutter. Indeed, the opening through which the fish has emerged would not be 

 detected by casual observation. Before hatching, the tail is curled around in a broad 

 arc so that its tip is above the head and extends into the region which later becomes 

 slit for the emergence of the young fish. It seems probable from observation and 

 inference that the fine extension of the tail acts like a flagellum and eventually wears 

 the case open in the slit-like manner described above. Apertures in the tendrils, through 

 which the water presumably circulates freely, are present before hatching; apparently 

 these "respiratory slits" do not exist at the time the egg is laid and we do not know 

 at precisely what stage of development they appear. 



I. J. Mar. biol. Ass. U. K., N. S. 12, 1920: 577-643. 



