(2) On the Formation of a Double Embryo in the Hen's-egg. 



By Professor W. Baldwin Spencer, M.A. 



Whilst working in the Biological Laboratory of the Mel- 

 bourne University, one, amongst a large number of hens' 

 eggs incubated, was found to present the curious feature of 

 having two clearly -formed embryos developed within the 

 limits of the one blastoderm. As this is by no means 

 of common occurrence, and as the embryos were quite 

 distinct, and developed to a considerable extent, it has been 

 thought worth while to figure them (PI. VI, fig. 2). Wolff, 

 Reichert, Thompson, and others have previously shown that 

 this may take place, and a figure showing two chick embryos 

 distinct from one another, is given by the latter investi- 

 gator.* In this case two embryos, of a very early stage, 

 each showing what is probably meant for the primitive 

 groove, are seen lying side by side, with their anterior ends 

 close together. They are not sufficiently developed to show 

 traces as yet of mesoblastic somites or nervous system. The 

 anterior ends of the two are closely approximated, whilst the 

 posterior ends diverge from one another. In the embryos 

 figured by myself, the blastodermic area is somewhat elon- 

 gated in the direction corresponding to that of the short axis 

 of the egg ; the area pellucida and the area opaca are clearly 

 distinguishable from each other and the latter is covered 

 with a complete network of blood-vessels, limited externally 

 by the sinus terminalis. The two embryos are so placed 

 that their anterior ends lie side by side in the middle of the 

 area pellucida, whilst their posterior ends are directed towards 

 the two opposite ends of the area. The two embryos are 

 precisely similar to one another. Each has reached the stage 

 at which the nervous system has the form of a tube, the 

 anterior end of which is becoming swollen out to form the 

 vesicles of the brain, though these are not as yet clearly 

 differentiated. At the posterior end of the body the neural 

 canal is still widely open, and encloses the remnant of the 

 primitive streak and groove. The head-fold has lifted the 



* I am indebted to Professor Allen, M.D., of the Melbourne University, for 

 the opportunity of seeing this. It is to be found in " Todd's CyclopEedia of 

 Anatomy and Physiology,'' under the article " Teratology." 



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