242 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



The bilateral symmetry of the salpa embryo is interesting from 

 another point of view, as it shows that, at a very early stage, imme- 

 diately after fertilization, if not before, the planes and regions of the 

 potential embryo are embodied in the structure of the egg in definite 

 relations to space. This property, which may, for convenience, be called 

 molecular organization as distinguished from cellular organization, may 

 be pictured as due to recent influences, or it may be imagined to be an 

 inherent property of the ovum, inherited from the ovary of the parent. 



Now all five eggs of Salpa hexagona lie, so far as their potential 

 organization goes, in the same plane, for the plane of the fertilizing duct 

 is also the plane in which each egg divides into a right and- a left half. 

 These eggs are greatly crowded at the time when they are shut into the 

 body of the chain-salpa, as this is developed in the wall of the stolon, and 

 as Plate XLV, Figs. 6 and 7, show, they are compressed and distorted 

 into very irregular shapes, and the five eggs in the set cannot, at this 

 time, maintain equivalent positions, nor can we suppose that they after- 

 wards right themselves and assume similar positions under the action 

 of gravity or some other influence which acts alike on all five, for they 

 are so closely wrapped by the surrounding follicle that they are not free 

 to move. So far as I can see, the only influence which affects all five eggs 

 in the same way is the spermatozoon, which, as it passes up the lumen of 

 the duct, must come into contact with the middle line of the proximal 

 surface of each egg. The first plane of segmentation is the plane which 

 passes through this point, and I think we are therefore justified in believ- 

 ing that the unfertilized egg of Salpa hexagona is not bilateral in inner 

 structure, but that the plane of fertilization becomes the plane of sym- 

 metry. So far as I am aware, salpa is the only animal in which the 

 definite plane of adult, bilateral symmetry can be proved to exist in the 

 unfertilized egg, and I think that there is good reason for believing that 

 it is not determined here by anything in the organization of the egg, but 

 by a purely accidental condition ; the fact that, as the opening of the 

 fertilizing duct is a fixed point, fertilization can take place at only one 

 spot. 0. Schultz believes (Zur ersten Entwicklung des braunen Froches) 

 that the middle plane is fixed in the frog's egg before fertilization, but 

 Roux (Beitrage zur Entwicklung-mechanic des Embryo, No. 4, Arch. f. 

 Mik. Anat. XXIX, 1887, p. 207) holds that while the poles are determined 

 by the distribution of the yolk, the egg of the frog is not differentiated 

 with reference to any particular one of the meridians which pass through 

 the poles, until after fertilization, when the plane of fertilization becomes 



