487 



mesoblast give rise to external markings that render the surface 

 appearances untrustworthy. There is, for instance, a deep pad of 

 mesoblast filling up a depression at the sides of the cerebellum, and, 

 also, extending forward on to the mid-brain ; at the external surface 

 the pad assumes a circular form, and it is that pad of mesoblast which 

 I have marked mb (mid-brain) in Fig. 4, p. 175 of the article refer- 

 red to. The true mid-brain vesicle in that Fig. is the eminence marked 

 A. op 1 (accessory optic vesicle). The accessory optic vesicles are 

 distinctly present, but they are in front of the protuberance marked 

 A. op 1 and are not well marked externally. 



In order to get a view of the brain walls, I have found it neces- 

 sary to remove the mesoblast and its covering layer of epiblast, and, 

 thus, to completely expose the brain walls. The brain thus laid bare, 

 enables one to see with complete satisfaction its different parts and 

 their relation to one another. A very interesting relation comes to 

 light through the dissections : The accessory optic vesicles are correctly 

 identified in Figs. 2 and 3, p. 172, but, somewhere between the stage 

 with an open neural groove (Fig. 3), and the completely closed groove 

 (Fig. 4), the mid-brain vesicle insidiously takes the former position of 

 the accessory optic vesicle, while the latter is carried forwards by the 

 process of cranial flexure. During the formation of the mid-brain 

 vesicle the two structures become incorporated into one faintly bilobed 

 protuberance. The anterior part is the accessory optic vesicle and 

 the posterior one the mid-brain. The separation soon becomes com- 

 plete, and by the stage represented in Fig. 4, the two are completely 

 separated, but, owing to the arrangement of the mesoblast, the accessory 

 optic vesicle is rendered indistinguishable from the outside. When the 

 mesoblast is entirely removed it may be readily seen. All this takes 

 place in a very brief interval of time, and a complete series of em- 

 bryos representing the different phases of the closure of the neural 

 groove is required to see it at all. Figs, are necessary to give any 

 satisfactory conception of the process, and I expect to publish soon, 

 a more complete an illustrated account, of the formation of the brain 

 vesicles and the changes in the roof of the thalamencephalon. In 

 the interim, I am desirous to call attention to my wrong identification 

 of the first formed mid-brain vesicle in the connection mentioned above. 

 It is to be understood also that the text is to be altered wherever 

 that part is referred to as the mid-brain. 



In another article (which appeared in this Journal No. 13) on 

 "Metameric Segmentation in the Medullary Folds and Embryonic Rim", 



