Manchester Memoirs, Vol. I. ( 1 906), No. 3- 



31 



dermal and a lower endodermal plate. To complete the 

 alimentary canal {^Fig. 15) the two plates of endoderm 

 first fuse by their inner edges below the notochord ; the 

 outer edges then bend down, meet, and so close the 

 central wall of the gut. 



A variation in the mode of formation of the notochord, 

 recalling the similar difference between the frog and the 

 lamprey, has been described by Henneguy in the trout ; 

 here the split in the roof of the archenteron passes right 

 across the middle line ; two cell plates are thus formed, a 

 lower, the endoderm, and an upper, which then becomes 

 differentiated into a median notochord and two lateral 

 mesodermal sheets. 



It is clear that the close similarity which obtains 

 between the Teleostei and the Elasmobranchii, in respect 

 of the closure of the blastopore, extends also to the 

 mode of formation of the germinal layers, 



GANOIDEI. 



Our knowledge is here unfortunately very slight. 



From the observations of Sobotta, Whitman and 

 Eycleshymer, and Bashford Dean, we know that in Ainia, 

 Lepidosteus, and Acipenser the segmentation of the ovum 



Fig. 16. 

 Formation of the germ layers in Acipenser (after Dean). 

 A. Sagittal section. B. Transverse section of dorsal half of embryo. 

 Lettering as before. 



