114 



to a minimum. But this is not the case, new ol)servations are con- 

 tinually forcing on the attention of embryologists the fact that transitory 

 organs arise and disappear without leaving any trace, and students of 

 development are just beginning to appreciate how essential it is to 

 have closer series of preparations, and to omit no period of develop- 

 ment however brief. 



I wish to give an account of transitory organs that arise and 

 disappear within a period of three hours or there-abouts. 



There is, in the head region of Vertebrate animals, a succession 

 of well known cerebral vesicles that arise in the neural tube of very 

 young embryos and undergo considerable and rapid transformations. 

 It has always been assumed that only one set of these vesicles arises, 

 and that they are cerebral vesicles in the true sense of the word. The 

 observations given below are opposed to this view. 



Having had occasion to examine with critical care the early stages 

 of chick embryos, I came to the conclusion two or three years ago, 

 that there are in that animal, at least two distinct sets of vesicles in 

 the early formed neural tube: a transitory series extending from the 

 24th to the 27th hour of incubation, and a later formed set, arising 

 about the 28th or 29th hour and extending onwards. The latter are 

 the well, known cerebral vesicles, and the former are problematical. 



In 1894, I observed a series of serially arranged vesicles (see 

 Fig. 2), extending along the sides of the neural tube directly back of 

 the eyes {op. v.) and developed on a ridge continuous with the latter. 

 The chick embryo had been so extensively observed, that it seemed 

 highly improbable, that these vesicles should have escaped mention, 

 and I should not now presume to draw the conclusion, that they are 

 new to morphologists , without adequate foundation. The history of 

 their rise, culmination and decline was at that time worked out and 

 studied in connection with the rise of the true cerebral vesicles. The 

 former set of vesicles is shown in Fig. 1 to 7, and the cerebral vesicles 

 in Fig. 8 and 9. Since 1894, I have had embryos showing these struc- 

 tures continually at hand, and, from time to time, have repeated the 

 original observations and allowed them to ripen, as we do Delafield's 

 hsematoxylin. 



The first traces of these structures appear when the embryo is 

 in the 24 -hour stage of development. There is slight individual 

 variation among the embryos, but, very close to that period, there is 

 to be detected an elongated pair of vesicles occupying each side of 

 the neural tube as shown in Fig. 1 op.R. These vesicles are lateral 

 expansions of the walls of the neural tube ; they are shallow grooves, 



