i \ri \\ \ i M>\ 01 ri \ : xliii 



'mexvhat 1 in \\hirli the primitive ciimiiln, i, till 



the opposite end () f the M.i,! i, which i, probably the 



rudiment of the pt<u eph.d'u ' 



Vii-w f an embryo from th- rfacc and from the side 



when six Moments ha\< -tablMied. 



Fig. 4. \'ic\v of an embryo, ideally unrolled, when the first rudiments of the 



appendages become visible. 



5. Kmbryo ideally unrolhxl at the stage when all the have 



become established. 



Fig. 6. Somewhat older stage, when the limbs begin to l>e joint- 

 from the side. 



Fig. 7. Later stage, viewed from the 



Fig. 70. Same embryo as fig. 7, ideally unrolled. 



Figs. 8rt and 8/>. View from the ventral surface and from the side of an embryo, 

 after the ventral flexure has considerably advanced. 



Fig. 9. Somewhat older embryo, viewed from the ventral surface. 



PLATES 31 AND 32. 



COMPLETE LIST OF REFERENCE LETTERS. 



ao. Aorta, ab. g. Abdominal nerve cord. eh. Cheliceroe. ch. g. Ganglion of 

 chelicene. <?/. Epiblast. As. Hemispherical lobe of supra-oesophageal ganglion. 

 ///.Heart. /. /. Lower lip. ///. Muscles, me. Mesoblast. mcs. Mesenteron. mp.g. 

 Malpighian tube. ms. Mesoblastic somite, ce. CEsophagus. /. c. Pericardium. 

 pd. Pedipalpi. pd. g. Ganglion of pedipalpi. /;-. Proctoda-um (rectum), pr. c. 

 Primitive cumulus, s. Septum in abdomen, so. Somatopleure. sp. Splanchnopleure. 

 st. Stomodaeum. su. Suctorial apparatus. sit. g. Supra-resophageal ganglion. 

 ///. g. Thoracic ganglion, v. g. Ventral nerve cord. y. c. Cells derived from yolk. 

 yk. Yolk. y. n. Nuclei of yolk cells. 



I g IV g. Ganglia of ambulatory limbs, i 16. Postoral segments. 



Fig. 10. Section through an ovum, slightly younger than fig. i. Shewing 

 the primitive cumulus and the columnar character of the cells of one half of the 

 blastoderm. 



Fig. ii. Section through an embryo of the same age as fig. i. Shewing the 

 median thickening of the blastoderm. 



Fig. 12. Transverse section through the ventral plate of a somewhat older embryo. 

 Shewing the division of the ventral plate into epiblast and mesoblast. 



Fig. 13. Section through the ventral plate of an embryo of the same age as 

 fig. 3, shewing the division of the mesoblast of the ventral plate into two mesoblastic 

 bands. 



Fig. 14. Transverse section through an embryo of the <ime age as fi-. 

 through an abdominal segment above and a thoracic segment below. 



Fig. 15. Longitudinal section slightly to one side of the middle line through an 

 embryo of the same ] 



