314 Thos. H. Montgomery. 



separate in origin, and the stomodaeal lips {Sto. L., Eig. 67b) are 

 simply ectoblastic thickenings into which extends the rostral meso- 

 blast. 



Other differentiations of the head lobe are the following: A short 

 distance anterior to the rostral appendages the head lobe is mesially 

 broadly indented, probably by its lateral borders growing more 

 rajDidly than its median, this constituting the anterior sulcus (Sul. 

 A., Fig. 64, PI. V). Behind this is the stomodaeum (Sto.) and 

 behind that the ventral sulcus (Sul. !>.), so that the head lobe is nearly 

 completely divided into right and left halves. The antero-median 

 margin of each half is somewhat elevated and thickened, and each 

 such transverse prominence, which may be called a cerebral ridge 

 (Ce. R., Figs. 63, 64) is bordered posteriorly by a groove, the fovea 

 (Fov.). A longitudinal section through a cerebral ridge and fovea 

 is given in Fig. G7a, PL VI. Fig. 68f shows a transverse section of 

 the two cerebral ridges ; in the midline lies the apex of the caud al 

 lobe (Caud.) and immediately above that the extraembryonic blasto- 

 derm (Ex., this being the plane of the anterior sulcus) ; right and 

 left of this sulcus are the halves of the head lobe, the mesial portions 

 of which are the cerebral ridges (Ce. R.). These ridges cannot be 

 considered separate appendages because they do not possess peculiar 

 miesoblast, but are bordered by the rostral mesoblast. Lateral from 

 and on a line with the fovea of each side is a transversely elongated 

 pit which may be termed the antero-lateral vesicle (A. L. Y., Fig. 

 63, PI. V) ; this is difficult to find on surface views, but on transverse 

 sections (A. L. Y., Figs. 68c, d, PI. VI) each is found to be an ecto- 

 blastic groove. In most of the head region the ectoblast is several 

 layers deep. 



The chelicera (Chel., Figs. 64, 65, PI. V) are at the posterior 

 border of the head lobe, as are their ganglia (Chel. G.), far behind 

 the stomodaeum (8to.). Their mesoblast sacs (Fig. 67a, PI. VI) 

 are separated from and much smaller than the rostral sacs. 



Each thoracal limb behind the chelicera possesses a distinct coelom, 

 bounded by somatic mesoblast extending into the limb and a splanch- 

 nic layer upon the yolk (Figs. 68d, e, 70). These mesoblast sacs do 

 not as. yet extend laterad of the limbs, but have grown some distance 



