566 LOCY. [Vol. XI. 



contours of the head are considerably altered by the distribu- 

 tion of mesoblastic cells lying between the external layer of 

 epiblast and the brain-walls. The mesoblast forms a pad of 

 varying thickness in close contact with the brain-walls; the 

 depressions are filled, and in some places thick patches of 

 mesoblast give rise to external markings that render the sur- 

 face appearances untrustworthy. There is, for instance, a 

 deep pad of mesoblast filling up a depression at the sides of 

 the cerebellum, and also extending forwards on to the mid- 

 brain ; at the external surface the pad assumes a circular form, 

 and it is that pad of mesoblast which is seen in Fig. 32. The 

 true mid-brain vesicle in that figure is the eminence marked 

 mb. The accessory optic vesicles are present, but are not 

 well marked externally. They are in front of the protuberance 

 marked 7nb. 



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

 necessary to remove the mesoblast and its coverings of epi- 

 blast, and thus to completely expose the brain-walls. The 

 brain thus laid bare, enables one to see with complete satisfac- 

 tion its different parts and their relation to one another. A 

 very interesting relation comes to light through the dissections. 

 Somewhere between the stage with an open neural groove 

 (Fig. 31) and the completely closed groove (Fig. 33) the mid- 

 brain vesicle insidiously takes the former position of the acces- 

 sory optic vesicle while the latter is carried forward by the 

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

 brain vesicle the two structures become incorporated into one 

 faintly bilobed protuberance (PL XXVIII, Figs. 38 and 39). 

 The anterior part is the accessory optic vesicle, and the pos- 

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

 plete, and by the stage represented in Fig. 40 the two are 

 completely separated ; but owing to the arrangement of the 

 mesoderm the accessory optic vesicle is rendered indistinguish- 

 able from the outside. When the mesoderm is entirely removed 

 it may be seen. All this takes place in very brief intervals of 

 time, and a complete series of embryos representing the differ- 

 ent phases of the closure of the neural groove is required to 

 see it all. I must say also that the changes are so perplexing 



