808 PROCEEDINGS OF THE AMERICAN ACADEMY. 



on. This membrane would be of great assistance, it seems to me, in 

 determining the origin of the mesoderm within the germinal area. 

 Many authors have not figured this structure, as, for example, Kolliker 

 ('82), Heape ('83), Bonnet ('84), Hubrecht ('90), and others. In the 

 rat and the mouse I have noticed a sharp line between ectoderm 

 and entoderm, which is probably the same structure, and it has been 

 figured by other investigators of these animals (see Duval '91, Robin- 

 son '92, and others). 



There is another point to which I wish to draw attention, without 

 however attaching too great significance to it. In the description of 

 my younger embryos I mentioned the fact that the germinal disk was 

 elliptical in outline, and that, according to my orientation, the shorter 

 axis of the ellipse lay in the plane of bilateral symmetry of the future 

 animal. Though it may be of little morphological significance, it is 

 certainly very interesting to note that in its earlier stages the blasto- 

 derm in teleosts (which must be held to be homologous with the ger- 

 minal disk of the mammalian embryonic vesicle) is also elliptical in 

 outline, and furthermore that the shorter axis of the ellipse corresponds 

 to the chief axis of the future fish, as established by Agassiz and 

 "Whitman ('84). This elliptical outline, which seems to be constant 

 in teleosts (see Ryder '84, Agassiz and Whitman '85 and '89, Wilson 

 '91, etc.), is produced in the first place by the first cleavage plane, 

 which divides the protoplasmic mass at the active pole of the egg into 

 two parts, each circular in outline, so that as they lie side by side the 

 blastoderm is elongated ; this condition persists for some time. 



If the first plane of cleavage in the teleost is not identical with the 

 plane of bilateral symmetry, my comparison, of course, has no validity. 

 So far as I am aware, there has been but one series of experiments 

 whose results would seem to disprove this theory ; and these were 

 conducted by Miss Clapp ('91), who worked on the eggs of Batrachus 

 tau. These eggs — attached by means of their thick outer membrane 

 to the vessel in which they were placed — were artificially fertilized, 

 and the position of the first plane of cleavage noted. Some seven days 

 later the chief axis of the future fish was clearly visible, and was super- 

 posed on the line of direction of the first cleavage plane. In only 

 three cases out of twenty-three did the two lines coincide ; in the rest 

 the second line made a greater or less angle to the right or left of the 

 first, — never greater than 70° however. But I think there is a possi- 

 ble source of error here, which makes my comparison still permissible. 

 The author states that rotation is impossible, since the yolk is at- 

 tached to the egg membrane at the point where the membrane attaches 



