. 61 



the same time lengthening and becoming more elliptical the 

 central cavity is much narrowed. 



At the same time it is observed that opposite to the above- 

 mentioned external opening, or to the spot where it was, the 

 surface of the ovum is raised up into two folds running 

 lengthways. At one end (the posterior extremity that is to 

 be) these folds curve round and join each other ; at the other 

 (anterior extremity) they gradually fade away. They are best 

 seen in transverse (optical) section, fig. 2, which shoAvs the 

 digestive cavity h, with its walls, still composed of large 

 cells, separated by the remains of the original cleavage cavity 

 c (now the general cavity of the body) from the body wall 

 composed of smaller cells, and raised. At w the body wall is 

 raised up into the above-mentioned folds which thus give 

 rise to the groove /. New cells may also be seen to have 

 made their appearance between the outer and inner sacs 

 beneath the folds. 



Although Kowalevsky was unable to trace out step by step 

 the arching over and coalescence of the (medullary) folds, he 

 believes that they do thus behave ; at all events an hour later 

 we meet with the form shown in fig. 3, in which h is, as 

 before the alimentary cavity, surrounded by its wall of cells 

 g, and c is the general cavity of the body. Outside the body is 

 now a new cavity, n, as yet opening externally at its front by 

 the orifice d, and roofed in by a double layer of cells originat- 



Fiq.3. 



Fici.l. 



ing apparently from the folds just spoken of. One of the 

 layers, Jc, forms the proper roof of the new cavity, while the 



