EXPLANATION OF PLATES I and II. 



List of Reference Letters, 



a. Anterior end. a. Anus. an. p. Anal papillae. At. Antennee. c. g. 

 Groove in brain, d. Dorsal ectodermal thickening, e. Eye. ec. Ectoderm. 

 en. Endoderm. i^. 1 . . . . Sfc. Feet. _;'. Jaw. j. s. Swellings at base of 

 jaws. L. Lip. M. Mouth, me. Mesenteron. or. p. Oral papilla;, o. s. 

 Opening of salivary gland, p. s. Praeoral somite. B. Rectum, s. 20. 20th 

 somite, s. g. Salivary glands, si. g. Slime glands. «. o. 4 and 5. Nephridia 

 of 4th and 5th legs. st. Ectodermal ingrowths into embryonic mouth. 



PLATE I. 



All the figures on this Plate, except 15, 17 — 22, are from fresh specimens. 



Fig. 1. — Per. Balfouri. Side view of unsegmented ovum, showing polar 

 bodies and dark patch. Greatest length *4 to 'iS mm. 



Fig. 2. — Per. Balfouri. Unsegmented ovum with dark patch, but with- 

 out central clear spot. 



Fig. 3. — Per. Balfouri. Unsegmented ovum with numerous dark patches, 

 each with a clear centre. 



Fig. 4. — Per. Balfouri. Ovum with two segments from side. 



Fig. 5. — Per. Balfouri. Ovum with four segments from animal polei 



Fig. 6. — Per. Balfouri. Side view of ovum with four segments. 



Fig. 7. — Per. Balfouri. Ovum with eight dark segments from anima 

 pole. Greatest length "4 to "48 mm. 



Fig. 8.— Per. capensis. Ovum fully segmented, with mosaic of ecto- 

 derm cells and scattered branched endoderm cells. Greatest length (of egg- 

 shell) "56 to '6 mm. 



Fig. 9. — Per. Balfouri. View of ovum from animal pole, as opaque 

 object. 



Fig. 10. — Per. capensis. Aggregation of the endoderm cells beginning. 



Figs. 11 and 12. — Per. Balfouri. Completion of same process. 



Fig. 13. — Per. capensis. Illustrates the same point. Length of ecto- 

 derm patch '32 to '4 mm. 



Fig. 14. — Per. capensis. Another phase of the same process. 



Fig. 15. — Per. capensis. Side view of ovum from preserved specimen, 

 Cap of ectoderm cells covering half the endodermal mass. Progress of 

 epibole. Diameter '240 mm. 



Fig. 16. — Per. Balfouri. Stage in which the endoderm cells are 

 covered by the flatter ectoderm cells. Diameter '32 mm. 



