ON A PLATYPUS EMBRYO, 



EXPLANATION OF PLATES. 



Beference LMtr><. 



am. a. Amniotic area. amc. r. Amuio-cardial vesicle, and. Auditoiy 

 plate, hi. Blastopore, hi. c. Blastoporic canal. hi. op. Internal opening 

 of blastoporic canal. h.r. Blood vessels ca^. Coelom. d. Jr. Median 

 sulcus of medullary plate ('" Riickenfurclie "). ect. Ectoderm, etit. Ento- 

 derm, eat ph. Pharyngeal entoderm. ?i.a. Heart Anlage. h. j^r. Head 

 process of primitive streak, hp. ones. Head plate of mesoblast. ht f:nd. 

 Heart endothelium, le. Longitudinal strand of cells lying just external to 

 the edges of medullary plate. m.s. Somite. m.s.c. Cavity of somite. 

 nid. f. Medullary fold, md, p. Medullary plate. me,H. Mesoderm. ■m<'>'. 

 ax. Paraxial mesoderm in front of Jst somite, mes. I. Lateral mesoderm. 

 N. Neuromere of mid-brain, n'-n.^'^ Neuromeres of hind-brain. nek. 

 Notochord. nch.c. Remains of notochordal canal, ojk gr. Optic groove. 

 pra. Proamnion. p7'. g. Primitive groove, /n: s. Primitive streak, so???. 

 Somatic mesoderm, sjil. Splanchnic mesoderm. I'as. r. Vasifactive cells. 

 vit. enf. Vitelline entoderm, w. h. Anlage of Wolffian body. m. d. Anlage 

 of Wolffian duct, i'k t. Anlage of Wolffian tubules. 



Plate IX. 



Photomicrograph of Platypus embryo from the egg just ready to be laid. 

 From the dorsal surface, (x St ) 



Plates x.-xiii. 



The positions of the sections from which figs. 1-13 are drawn are indicated 

 in the photo-micrograph by corresponding numbers. 

 All the drawings were made with a Zeiss' camera lucida. 



Plate X. 



Fig. 1. — Transverse section through the anterior end of the medullary plate, 

 showing its separation from the vesicle, the proamnion {pra.) 

 underlying it and the amnio-cardial vesicles {amc. v. ) laterally, 

 (x 70.) 



Fig. 2. — Transverse section of the middle region of the future fore-brain 

 passing through the commencement of the optic groove [op. gr.). 

 The median sulcus {d.fr.) will form the future medullary groove 

 proper. ( x 70.) 



Fig. .3. — Transverse section slightly posterior to fig. 2 showing the hollow 

 outgrowth from the floor of the optic groove. { x 70.) 



