NOTES ON OVULATION, OVA, AND EARLY DEVELOPMENT IN THE SMOOTH DOGFISH, MUSTELUS CANIS

¹ Department of Zoology, Smith College and the Marine Biological Laboratory, Woods Hole, Massachusetts

1. The ovum and egg-case of Mustelus canis, a viviparous elasmobranch, are described and the orientation of the egg-case in utero is noted.

2. The blastoderm is typically located near the cloacal end of the yolk and the dorsal lip of the blastopore forms at the anterior border of the blastoderm when the egg is oriented as it lies in utero. Occasional cases of reversed and oblique orientation are found.

3. In early June, mature females are post-partum. The functional right ovary contains large ova soon to be ovulated while the left ovary is vestigial.

4. The ovulation period extends from about June 20 to July 20. Four to eight ova are generally found in each uterus of a pregnant female and, from a comparison of stages present, it is probable that as a rule two ova are discharged simultaneously with an interval tentatively estimated at 30 to 40 hours between most subsequent ovulations.

5. Ova are arranged in sequence in each uterus with the most advanced embryo nearest the cloacal end.

6. Stages of embryos present in utero between June 23 and Aug. 31 are tabulated. An egg ovulated at the beginning of this period would develop into an embryo with a maximum length of about 80 mm. in 9 weeks time.

7. From observations made on living material, blastoderms with thickened dorsal lips (gastrulation stages) are estimated as approximately 5 days younger than fused neural tube stages 3.5 to 4 mm. in length.

8. On the basis of egg-cases devoid of yolk as well as from the occurrence of "double-yolked" eggs, it is suggested that oviducal glands are controlled by ovarian hormones and that a single egg-case is secreted simultaneously by both right and left glands irrespective of the number of ova discharged at a given ovulation.