I'l \NATION 01 MAI I i i. xix 



Cenninal epithelium. ////>. Musrlr pl.it.-. 



band of muscle-^ fnun the splamhnir 1. ' muscle-plates, nt. Neural canal. 



M IK >iu y..lk in i ivity. po. 



live ovum. //. l: inu-nt of p>Mn : pinal nerve, td. 



:ital duct, sh. Cuticular sheath of notorh'.nl. .. ><,m.iti< li\, r of mesoblatt. 

 .v/>. Splanchnic layer of mcsoblast. spc. rd. sp. v. Spiral valve, jr. Inter* 



renal body. st. Segmental tube. sv. Simi- veno>u>. // ,/. Umbilical artery, urn. 

 Umbilical cord. ttr. Umbilical vein. v. Splanchnic vein. v. Blood-vessel, vc. Visceral 

 cleft. Vr. Vertebral rudiment. //'. \Vhitematter of spinaK -chorda! 



rod (except in fig. 14 a), y. Passage connecting the neural and alimentary canals. 



I'l.ATK II. 



Fig. i. Section from the caudal region of a Pristiurus embryo belonging to stage 

 H. Zeiss C, ocul. i. Osmic acid specimen. 



It shews (i) the constriction of the subnotochordal rod (x) from the summit of the 

 alimentary canal. (2) The formation of the body-cavity in the muscle-plate and the 

 ventral thickening of the parietal plate. 



Fig. i a. Portion of alimentary wall of the same embryo, shewing the formation 

 of the subnotochord rod (x). 



Fig. 2. Section through the caudal vesicle of a Pristiurus embryo belonging to 

 stage H. Zeiss C, ocul. i. 



It shews the bilobed condition of the alimentary vesicle and the fusion of the 

 mesoblast and hypoblast at the caudal vesicle. 



Fig. 3 a. Sections from the caudal region of a Pristiurus embryo belonging to 

 stage H. Zeiss C, ocul. i. Picric acid specimen. 



It shews the communication which exists posteriorly between the neural and 

 alimentary canals, and also by comparison with 3 b it exhibits the dilatation undergone 

 by the alimentary canal in the caudal vesicle. 



Fig. 33. Section from the caudal region of an embryo slightly younger than 30. 

 Zeiss C, ocul. i. Osmic acid specimen. 



Fig. 4. Section from the cardiac region of a Pristiurus embryo belonging to stage 

 H. Zeiss C, ocul. i. Osmic acid specimen. 



It shews the formation of the heart (ht) as a cavity between the splanchnopleure 

 and the wall of the throat. 



Fig. 5. Section from the posterior dorsal region of a Scyllium embryo, belonging 

 to stage H. Zeiss C, ocul. i. Osmic acid specimen. 



It shews the general feature- of an embryo of stage II, more especially the rela- 

 tions of the body-cavity in the parietal and vertebral portions of the lateral plate, and 

 the early-formed band of muscle (/;//) in the splanchnic layer of the vertebral plate. 



Fig. 6. Section from the u-Miphagial rei;i"ii of Scyllium embryo belong 

 stage I. Zeiss C, ocul. i. I'hromic aci.l .specimen. 



It shews the formation of the rudiments of the posterior nerve-roots (pr) and of 

 the vertebral rudiments ( Vr). 



Fig. 7. Section of a Torpedo embryo belonging to stage slightly later than I. 

 Zeiss C, ocul. I, reduced J. Osmic acid specimen. 



