386 C. 0. WHITMAN. 



primitive groove. The primitive groove is simply " a phe- 

 nomenon of conjunction,'^ as Kauber terms it; but it differs 

 from the medullary groove in not being confined to a single 

 embryonic layer. Both fall in the same median plane, but one 

 is primary, the other secondary ; one divides the entire embryo 

 into two homo-typical halves, the other only marks the middle 

 line of the neural plate. These are, of course, no new facts ; 

 but I have been obliged to state them in order to make clear 

 my meaning. Now, if we bear in mind that the primitive 

 groove is only a seam that marks the incomplete coalescence of 

 two germ bands or halves of the germinal ring, and that the 

 medullary groove simply marks the axial relations of two folds 

 on the surface of these bands, we may avoid the confusion that 

 is sure to arise when this distinction is not kept in view. Re- 

 garding this distinction as the only one of fundamental import- 

 ance, I hold that all disputes in regard to the presence or 

 absence of a primitive groove do not affect the main question 

 of concrescence. The plane of junction may be marked by a 

 narrow surface groove or only a faint line, or the coalescence 

 may be so complete that no plain indication of its position can 

 be detected. In all Amniota the two grooves are not contem- 

 poraneous in origin, and one is obliterated as the other takes 

 its place ; but in Elasmobranchii they are coincident, a circum- 

 stance that does not alter the fundamental distinction before 

 insisted upon. 



The aim of these remarks, if it need be stated, has been to 

 show that the manner in which the medullary folds close is 

 not of primary importance in deciding between the theories of 

 differentiation and concrescence. It is possible, and perhaps 

 not improbable, that the formation and closure of the medullary 

 folds are determined in part by the same causes that concur in 

 bringing together the two halves of the germinal ring; but the 

 two closures are nevertheless quite distinct, both from a mor- 

 phological and physiological stand-point. The concrescence 

 theory undertakes to account for the conversion of the germinal 

 ring into a bilateral embryo ; but it is not within its scope to 

 explain peculiarities in the closure of the medullary folds, since 



