178 RECORDS OF THE AUSTRALIAN MUSEUM. 



The most striking feature of the fcetus is the peculiar yolk-sac. 

 It is not globular like the sac of all other forms I have seen, but 

 greatly elongate, its length being more than twice its major dia- 

 meter ; it is also larger at one end than the other. The entrance 

 of the umbilical vessel is neither terminal nor median, but to the 

 side of the larger end, its position being very similar to that of 

 the oesophageal entrance at the cardiac end of the human stomach. 

 The umbilical vessel is slender and of great length, its diameter 

 is not more than 2 mm., while its length is 78 mm. or exactly 

 half that of the total length of the fcetus. The colour of the yolk, 

 seen through the investing membrane, is a bright orange, precisely 

 similar to that of a fowl's egg. 



The significance of the singular shape of the yolk-sac and the 

 length of the umbilical cord, is not difficult to comprehend. When 

 first formed, the sac tills its section in the uterus ; as the foetus 

 develops it becomes longer than the chamber, and the tail curves 

 round. A space is thus formed, wider behind where the bend 

 occurs, and narrower in front where the tail touches the body. 

 This space is occupied by the yolk-sac, to which its shape exactly 

 adapts it. 



The umbilical cord, entering the yolk-sac at the larger or hinder 

 end, requires to be of great relative length to reach the umbilicus 

 of the fcetus. As the Shark still further increases in size, the 

 umbilicus travels backwards (in relation to the chamber) and a 

 shorter cord is therefore necessary. The shortening of the cord 

 is also favoured by the shrinkage of the yolk-sac and the move- 

 ments of the fcetus, until at birth, when the sac is smaller than a 

 pea, the length of the cord is not remarkable. 



