The Embryology of Chlamydoselachus 593 



drawings. Each figure will be compared point by point with the next younger in order to 

 show the relative progress in development. Then so far as possible, comparisons will be 

 made with embryos of Squalus acanthias of about similar si2;e as portrayed so well in 

 Scammon's "Normal Plates" (1911). 



An Embryo of 11.5 Millimeters 



This is the smallest embryo listed and figured. It and two other small embryos 

 (15.5 and 20 mm.) were taken "1905 Early January." Presumably all came from the same 

 mother. If so, this shows that the embryos and eggs in a given uterus may be of different 

 but closely related ages. This is to be expected since the eggs presumably ripen one at 

 a time and are discharged from the ovary singly; certainly they pass one at a time into the 

 oviducal funnel, are fertilized and encapsuled as they pass down into the uterus. FertiH' 

 nation of these shelled eggs must take place before the capsule is formed. In Ginglymostoma 

 I have taken segmenting eggs from the oviduct above the shell gland. 



This embryo, seen in right lateral aspect, is labelled in the original drawing "Emb. 

 C 11. 5 mm.", and is shown in Figures 15 and 16, plate II. Figure 15 bears the notation 

 '■'■X10+", and in the original drawing it measures 121 mm. Figure 16 is drawn to larger 

 scale. In length this embryo corresponds to Scammon's (1911) Fig. 24 of an 11.5'mm. 

 Squalus acanthias shown in his pi. II. In development the two embryos are in about the 

 same stage. As my Figure 15, plate II, shows, the frilled-shark embryo is attached to 

 the large yolk sac by a short yolk stalk. In the original drawing this has an antero- 

 posterior diameter of 10 mm. but in life of about 1 mm., which is the measurement for the 

 cord of the 11.5-mm. Squalus. This is the "umbilical cord" of Nishikawa (1898). How- 

 ever, this is not an umbilical cord but merely a yolk stalk. 



On the dorsum of this frilled-shark embryo (Figure 15, plate II), there is a convexity 

 over the gill-arch region, a concavity over and behind the vertical of the yolk stalk, 

 a sHght convexity behind this, and a marked downward bend of the tail. The forebrain 

 and midbrain make an angle of approximately 90° with the main axis of the body. The 

 forebrain looks downward. The midbrain is delimited from the forebrain by a groove and 

 superficially is sharply marked off from the hindbrain. The nasal groove, having its 

 greatest invagination in front, is placed just below the eye. The optic cup is circular and 

 without trace of optic fissure. The lens is prominent and circular in outline. The mouth 

 is widely distended. The gill-plate is prominent, showing seven branchial grooves — the 

 first indistinctly. Not all the grooves appearing in this region are branchial grooves. The 

 second branchial groove appears to be forked, but the anterior limb is not a branchial 

 groove as may be seen by comparing this drawing with Figure 16 where the gill-clefts 

 are shown in larger scale. 



The pectoral-fin rudiment is well-outlined for this early stage, and extends backward 

 and downward, reaching some distance back of the hinder edge of the yolk cord. The 

 cloacal elevation is quite prominent and in front of it is a sHght swelling which I take to be 

 the anlage of the pelvic fin. The tail ends in a point bent sharply downward. The 

 V-shaped myomeres show faintly in the upper half of the trunk. 



