Fis/ies of the JVester?i North Atlantic 189 



considerably lower percentages of pregnancy in other months of the year; in these 

 latter waters the percentage of pregnancy is always much lower than that in Block 

 Island Sound fish. There is considerable variation from sample to sample, but in general 

 the above pattern is remarkably consistent. We believe that the seasonal nature of 

 breeding as outlined is the true situation, and we have much evidence to show that 

 this picture does not arise from the vagaries of sampling by commercial fishing vessels — 

 i. e., the tendency to fish possible spawning grounds of Skates in certain seasons (June- 

 July and November— January) and to avoid those grounds in other months. Incidentally, 

 in this regard the percentage of females with uterine wounds shows no striking cor- 

 relation with the seasonal nature of productivity, although there may be a slight increase 

 in the' percentage two to four weeks before the peaks of productivity. 



Ovulation takes place in both ovaries at approximately the same time, and the 

 formation of the egg case by each of the two shell glands begins before either of the 

 eggs has reached that level in its passage down its respective oviduct. By the time the 

 cases are approximately V4— Vs formed, single eggs enter each after being much elongated 

 in passing through the shell gland. Following the completion of egg-case formation, 

 activity in the right arm of the genital tract invariably precedes that in the left arm. 

 Thus, while eggs are always prepared for laying in pairs, the right egg descends and 

 reaches the uterus first and is laid slightly in advance of the left egg (two exceptions 

 out of thousands examined); we have detected no anatomical difference to account for 

 this phenomenon. At one end the egg case always has two long straight tendrils, at 

 the other end two shorter tendrils whose terminal portions are likely to be recurved; 

 the end with the longer tendrils is laid down first by the shell gland. As a result of 

 aquarium experiments and palpating techniques, we find that egg cases are formed 

 and laid in a surprisingly short period of time, the entire process in June and July 

 occurring in 48—72 hours. In the winter months the process is undoubtedly slower, 

 and a female may retain fully formed egg cases for as much as three weeks. It appears 

 that a few eggs in each ovary become ripe at approximately the same time and that 

 these are laid in pairs at intervals from as little as five days to several weeks in the early 

 summer production peak. Thus one of our aquarium fish laid four pairs of egg cases 

 between June 7th and July 1 2th, i 950. Whether a mating is requisite for the fertilization 

 of each successive pair of eggs or whether one copulation may serve to effect fertilization 

 in several successive pairs of eggs is not known, since we had both sexes in our ex- 

 perimental tanks. Following the period of egg-laying, it appears likely that the whole 

 genital tract undergoes somewhat of a regression, probably prior to building up the 

 ovaries to a condition where they once again contain ripe eggs {vide infra). On the 

 other hand, adult males carry sperm continuously at all times of year from the onset 

 of maturity to death. 



In our tank experiments, 12 females produced 65 egg cases between May 30th 

 and July 12th, 1950, an average of over three pairs per fish. Of these 65 egg cases, 

 only 10 (15 %) were found to contain living embryos; the others were either infertile 

 or failed to survive past an early stage of development. This low percentage may have 



