426 



diffuse condition eventually becomes that part of the ectoderm lying in 

 the mid-ventral line. 



The late appearance of the mesendoderm in Holopedium is remark- 

 able. At the stage when it is being differentiated (Fig. 3) the rudiments 

 of the second antennae are already well advanced, and the rudiment of 

 the carapace has appeared. 



Fate of the Yolk. 



Fairly late stages show occasional very flat nuclei lying on the 

 separate yolk masses, as figured by S ama s s a. Doubtless each yolk 

 mass is contained in a single yolk cell. The origin of these yolk cells 

 has not been observed, but it may safely be assumed that they arise in 

 the same way as that described by Samassa — i. e. by budding off from 

 the mesendoderm. None of the nutritive material of the egg ever comes 

 to lie in the alimentary canal. 



Larval Cuticles. 



The extremely early appearance of the first cuticle is very remark- 

 able. My reasons for considering it a cuticle rather than a vitelline 

 membrane or an egg shell secreted by the oviduct and for considering 

 the casting of it an ecdysis rather than hatching are as follows: — 



The diameter of the oviducts is very much less than that of the laid 

 egg, and consequently the ripe eggs lying in them are compressed into 

 an elongated cylindrical shape. Up to and including the moment of 

 laying the egg must remain very soft and pliable, in order to allow it 

 being passed through the narrow opening of the oviduct. The spherical 

 shape is not assumed till after the egg is laid, so that at any rate, the 

 egg if provided with a membrane at all, must have only a very thin and 

 flexible one. As a matter of fact I have not been able to discover a 

 membrane round the oldest eggs I have seen in the ovaries. These were 

 however not quite ready to be laid as shown by the fact the three "nurse 

 cells" which accompany each egg cell are still present though reduced 

 in size. 



The cuticle seen in the 16-cell stage is tough and of a considerable 

 thickness, and is a conspicious object in sections. So whether the egg 

 in the oviduct has a very thin membrane or not, it is certain that the 

 bulk of the cuticle seen in the 16-cell stage, and certainly all the chitinoid 

 stiffening has been secreted after laying. The secretion and ecdysis of 

 the chitinoid cuticles of Arthropods has long been recognised as a means 

 of nitrogenous excretion, and many cases are known of embryonic 

 ecdyses. The cuticle of the segmenting egg of Holopedium may there- 

 fore be regarded as a true Arthropod cuticle, and the casting of it at 



