114 Proceedings of the Royal Society of Victoria. 



liead up above the blastoderm, but the latter is not yet 

 enclosed by the amnion. In the middle region of the body 

 seven pairs of mesoblastic somites are present.* 



The vessels passing across the area pellucida to the 

 embryos are not yet clearly visible, and the sinus terminalis 

 is perfectly complete, no such structures as the one or two 

 large vessels being present, which in the normal embryo 

 leturn the blood to the body from the sinus at the anterior 

 end. 



Apparently, every stage may be met with between an 

 embryo which shows reduplication of one portion of the 

 body and the condition in which, as described above, two 

 perfect embryos are formed within the area of the same 

 blastoderm. One of these stages is represented in the 

 adjoining drawing (Fig. 1), by Mr. Lucas, of an abnormal 

 embryo, showing a clearly double formation at the head end 

 and an indication also of doubleness at the posterior 

 extremity, where the primitive streak divides into two 

 halves, running out right and left of the median line. 

 Occasionally an embryo is met with showing only this 

 double nature of the primitive streak posteriorly. 



Regarding simply the case in which the two embryos are 

 complete and separate from one another, there are perhaps 

 three ways in which it might possibly be supposed that the 

 result has been brought about. First, as in the case of 

 Lumbricus trapezoides division of the, at first single and 

 normal, embryo may have taken place after a certain stage 

 of development has been reached. It is difficult to imagine 

 how this could have been produced ; had it been so, the yelk 

 and the area pellucida and opaca would have shown some 

 trace of division. Secondly, it might be supposed that the 

 two embryos were due to the existence of two distinct 

 nuclei, enclosed abnormally within the protoplasmic material 

 constituting one ovum. In contrast to the usual method 

 of formation of the germinal cells in Craniata out of a 

 number of nucleated cells, which first become aggregated 



* For teaching irarposes, I have found it convenient to reUuquish the old 

 form of nomenclature according to which chick embryos were designated by 

 the number of hours of incubation — a most unsatisfactory method, since 

 different eggs incubated for the same length of time will often yield embryos 

 of various stages of development. I have instead adopted the method 

 followed by Balfour in dealing with elasmobranch embryos, and according to 

 which the stages of development are indicated by the letters of the alphabet. 

 An account of these stages, with illustrative figures, is now in course of 

 publication. The embryos here referred to are at the commencement of the 

 stage which will be designated G. 



