80 BULLETIN OF THE 



wards and medianwards of the endoderm and the mesenchyme results in 

 the closure of- the endoderm to form the gut at the same time with the 

 union of the two vessels to form the heart. In the first type^ the endo- 

 thelial lining of the heart is formed by a group of cells which are enclosed 

 by the right and left plates during their union ; in the second type, the 

 lining is already present as a more or less tuljular structure in each of 

 the two blood-vessels before they unite. 



I have endeavored in what follows to present an account of the early 

 stages in the development of the heart in the cod, supporting my con- 

 clusions by additional evidence from other bony fishes, and have also 

 given an account of the methods which' have seemed to me to bo most 

 useful. 



I wish to acknowledge my indebtedness to Dr. E. L. Mark for his kind 

 supervision of my work, and to Mr. E. B. Hodge of the New Hampshire 

 Fish Commission and Mr. E. E. Boyer for generously aiding me with 

 material. 



Methods and Technique. — Handling and Care of Fish Eggs. 



The proper treatment and care of the eggs is a matter of the utmost 

 importance in studying the embryology of fishes. The cod, fundulus, 

 and shad material which I used had already been carefully prepared and 

 preserved ready for staining and sectioning, but I also made use of the 

 eggs of lake trout, brook trout, and salmon, which were received alive. 

 Eggs of the brook trout can be taken from the middle of September to 

 the middle of November, those of lake trout and Penobscot salmon (Salmo 

 salar) during the latter part of October and through November, while 

 those of the land-locked salmon are best taken later in November and 

 in December. 



The cod material which I used was taken in December, the fundulus 

 in July, the shad in the latter part of May and first part of June. 



The method of transporting fish eggs during young stages has received 

 considerable attention from the United States Fish Conmiission ; the 

 method adopted for handling small quantities is as follows. A bed of 

 damp moss containing a quantity of chopped ice is made in a perforated 

 box or pail, and on this bed are laid a few folds of damp mosquito net- 

 ting upon which the eggs are placed. The eggs are then covered by ad- 

 ditional folds of damp netting, and on this in turn ai'e placed more damp 

 moss and ice. Although several layers of eggs can be arranged in this 

 way, -the pressure from above must nut be too great. This arrangement, 



