112 Proceedings of the Royal Society of Victoria. 



than the other. Both are fully developed up to date ; the 

 unseefmented brain beino; carried over the front of the head 

 fold producing the mitriform appearance. The (optical) right 

 neural fold of the right head is continued back along the 

 common trunk, and so the left neural fold of the left head. 

 The adjacent neural folds can only be traced to the level of 

 the point of divergence of the fork, and seem to meet as 

 they thin out. They are quite wanting in the trunk. 



There are eight rows of mesoblastic somites with definite 

 outlines, and traces of others can be discerned. The outer 

 row on each side is normal, and close to the corresponding 

 neural fold. The two inner rows have fused to form com- 

 pound median somites, which have double the width of the 

 single outer ones. The hinder part of the primitive streak is 

 double, the two divisions running outwards at right angles 

 to the main axis of the embryo. The vascular area and the 

 primitive heart seem to be quite simple. The specimen was 

 mounted as a whole, and I am unable to distinguish the 

 hypoblastic structures. 



The double form may have been produced in two ways. 

 The product of a single germinal vesicle may have undergone 

 a certain amount of longitudinal dehiscence, or the products 

 of two germinal vesicles may have partially fused in the 

 growth side b}^ side, on a yolk originally common to both, 

 or formed by union of the two yolks. The process, 

 whichever it has been, has affected all the three germinal 

 layers at the two extremities, where the separation of the 

 two embryos is complete. In the region of the somites, the 

 somatopleure is distinctly divided into two equal longi- 

 tudinal halves, which have remained in juxtaposition ; the 

 splanchnopleure does not show any signs of fission. 



I do not think that there is any evidence upon which we 

 can decide in which of the two ways the doubling has been 

 brought about, but it is plain that the cause must have been 

 deep-seated, and must be looked for in far earlier stages. 



