MUSEUM OF COMPARATIVE ZOOLOGY. 83 



My "best results were secured with materitd killed in picro-sulphuric acid. 

 The embryos were left for three hours in the fluid and then removed and 

 dehydrated by passing them slowly through successively stronger grades 

 of alcohol from 35% up to 90%. The egg membranes were then carefully 

 punctured or cut. 



In staining, the eggs were passed through weaker grades of alcohol and 

 then stained for twenty-four iiours in Czokor's cochineal. 



I found it best to section without attempting to remove the membrane 

 or any of the hardened yolk. The membrane of shad eggs can easily be 

 removed, but that of the other eggs which I studied is so close to the 

 embryo as to make it more or less dangerous to attempt its removal. In 

 endeavoring to remove hardened yollv, too, there is the greatest danger 

 of harm to the embryo by the cracking of the yolk, or by straining it in 

 such a way as to brealv or rupture the embryo, which is very brittle. 



Great difficulty is experienced in sectioning the material, for parts of 

 the yolk break up and fall out of the section, often carrying the embryo 

 ■with them, while the firm membrane is apt to resist the knife, and fold 

 or break the embryo. A good plan is to have a generous amount of the 

 imbedding substance about the object, and to cause the knife to strike 

 the embryo before it docs the yolk. Thus there is less danger that the 

 embryo will be torn or pushed by the resisting yolk. 



The Development op the Heart ix the Cod (Gadus viorrhva). 



The heart of the cod is formed, like that of other Teleosts, in accord- 

 ance with tlie first type referred to in the Introduction, that is, by tlie 

 union in the median plane of the lateral plates of mesoderm. The for- 

 mation takes place slightly posterior to the ear capsule, and with the 

 average rate of development is completed between the ninth and tenth 

 days. 



The lateral plates can be distinguished even during the second and 

 third days, but only in a region' posterior to that in which the heart is 

 about to be formed. During this early period, the lateral plates, owing 

 to the sinking down of the embryo into the 3'olk, are folded up in such a 

 manner that tiicy are almost vertical and lie pressed against tlie proto- 

 vertebne, instead of being spread out ilat as in most otlier Vertebrates, 

 and as they arc at later stages in the development of the cod (Plate I. 

 Figs. 2 and 4). At this stage the endoderm below the chorda is being 

 folded to form tlie primitive fore-gut (Fig. 2). lichinri the region of the 

 heart the gut is alieady closed, Ijut in the heart region the margins of 



