iil:vi<:l()pment of planula in a pi-umulaimax iiydiiMhh. lol 



eududerm and those of tlie " coveriug cap "' have becouie very 

 elongated and attenuated. 



Fig. 1(1. — X 220. Longitudinal section at the next stage. The distal 

 endoderm and the ''covering cap" have completely disappeared and tlie 

 ovum is surrounded by cells which have arisen from the scattered ecto- 

 derm of the broken-down gonophore. The ovum is being pushed into 

 a central position by the growth of the diverticulum of the coilenteron 

 (d. C.) which is the endodermal spadix of the gonophore. 



Fig. 11. — X 220. Longitudinal section showing embi-yo (M.) derived 

 from the segmentation of the ovum. It consists of about sixteen cells, 

 and is central in position immediately below the middle point of the 

 distal surface of the perisarc of the gonotheca. The embryo is sur- 

 rounded by the placental tissue. The diverticulum of the ccelenteron 

 (d. C.) or gonophore spadix has become equal in size to the terminal 

 portion {t. p.) of the ccelenteron of the gonangium. 



Fig. 12. — x 480. Enlarged view of morula imbedded in placental 

 tissue {pi. t.). 



Fig. 13. — x 220. Later stage. The eml>ryo. the placental tissue, 

 and the ccelenteron pouches have grown. The embryo has developed a 

 segmentation space (sg. C). 



Fig. \i. — X 220. In the next stage, a differentiation of the cells of 

 the emljryo into an outer ectoderm and an inner endoderm is seen. 

 Distally there is only ectoderm {ect.^) which is very thin ; proximally the 

 ectoderm is thicker and is more than one cell thick. 



Fig. 15. — x 220. The emliryo becomes pear-shaped, and the endo- 

 derm is not continued distally. The maternal ectoderm (e E.), derived 

 oi'iginally ivom. the placental tissue, is continued around the outside of 

 the coilenteron pouches, and the cells l)ecome very elongated and reach 

 the lateral perisarc of tlie gonotheca. 



Fig. 16. — x 220. A later stage where the embryo has grown con- 

 siderably and has extended inwards towards the centre of the gonangium. 

 The ccelenteron pouches, consisting, on one side, of the spadix of the 

 gonophore, and on the other, of the terminal portion of the ccjelenteron of 

 the blastostyle, form a deep cup containing the embryo and the true 

 placental tissue {pi. t.). The enveloping ectoderm (c E.) has also 

 greatly extended. Bl. is blastocrel. The endoderm {g. e.) of the 

 gonangium at the base of the embryo is more granular than elsewhere. 

 The endoderm of the embryo now forms a contimious layer. It is 

 mostly more than one cell thick, as also is tlie outer ectoderm. 



Fig. 17.^ — X 140. Enlarged view of a piece of embryo, the surround- 

 ing placental tissue and the endoderm of gonangium. The ectoderm 



