142 DISCOVERY REPORTS 



were fixed in saturated mercuric chloride and 5 per cent acetic acid solution. The excess 

 mercuric chloride was washed out in iodine alcohol before storage in 70 per cent spirit. 

 Thirty-eight Gentoo embryos were collected in South Georgia in the 1 930-1 season, 

 together with the Ring penguin embryos from the South Shetlands. These were 

 preserved in Bouin's fixative and were also received in 70 per cent spirit. Both batches 

 of material arrived in splendid condition ; the corrosive-fixed embryos proved the better 

 material for sectioning purposes, while the Bouin-fixed embryos were the better for 

 dissection. 



Owing to the method of collection there is no information about the precise length of 

 time that any particular embryo has been incubated. 



STAGES OF DEVELOPMENT 



The embryos make up a remarkably complete series of stages, extending from a small 

 Gentoo embryo 3-8 mm. in length to a large Ring embryo on the point of hatching that 

 measures 150 mm. from the tip of its beak to the end of its tail. 



The earlier stages, which are all of Gentoo embryos, bear so close a resemblance to the 

 corresponding stages in the fowl that it will be convenient to define them in terms of the 

 Normentafel of Keibel and Abraham. 



1 . The smallest embryo (Plate I, fig. i) compares with a fowl of 24 hours' incubation. 

 NT (i.e. Normentafel), stage 9. 



Length, measured from the anterior limit of the medullary folds to the posterior limit 

 of the primitive streak, 3-8 mm. Length of primitive streak loooju. 



Medullary folds dilate anteriorly to an exceptional degree and widen posteriorly 

 around the headward end of the primitive streak. The folds are not continuous behind 

 the primitive streak as they are in Peripatiis and Lepidosiren (cf. Kerr, 19 19, pp. 496, 

 497). Number of mesoderm segments 6. 



Notochord {utc), visible for a considerable distance through the floor of the neural 

 canal, terminates anteriorly in a Y-shaped split. 



Fore-gut {fg) developed symmetrically. Extends from the headward end of the 

 division of the notochord nearly to the level of the first mesoderm segment. 



Proamnion lies in front of the head fold, and precociously developed cavities of the 

 splanchnocoele containing fluid, at the sides of the fold, are conspicuous features of the 

 embryo at this stage. 



2. The next stage is comparable with a fowl of 48 hours' incubation (Plate I, fig. 2). 

 NT 13. Length 5 mm. 



Medullary folds closed anteriorly, head already bending towards the ventral side. 

 Optic rudiments established. Otocysts also clearly visible. Number of mesoderm seg- 

 ments 18. 



Heart S-shaped. Aortae {ao) paired anteriorly, give off right and left vitelline arteries 

 posteriorly. 



Embryonic excretory organ established in a rudimentary condition. It may be seen in 



