260 LILIAN SHELDON. 



appear to be segmented, but this may be due to the action of reagents. This 

 figure was compounded from two sections. Pm. S. Protoplasmic segments, 

 r. Yolk. ?e. Nuclei. Oc. 2, obj. CO. 



Fig. 11. — Transverse section through the middle of an ovum, in which 

 there is a special area of protoplasm at one pole forming a reticulum, in 

 which many nuclei lie. Nuclei are also present scattered through the yolk. 

 The yolk is segmented. Pm. A. Protoplasmic area. Y. S. Yolk segments. 

 Oc. 2, obj. A. 



Fig. 12. — The protoplasmic area of the ovum shown in Fig. 11, more highly 

 magnified, to show the reticulate arrangement of the protoplasm, the absence 

 of cell outlines, and the irregular arrangement of the nuclei. Oc. 2, obj. D. 



Fig. 13. — Transverse section of an ovum, rather older than that from which 

 Fig. 11 was drawn. The protoplasmic area (Pm. A.) has become more com- 

 pact and flattened. The nuclei in the rest of the egg are more numerous 

 round the periphery than in the centre. The ovum is broken at two points. 

 Pm. A. Protoplasmic area. V. m. Vitelline membrane. Oc. 2, obj. A. 



Fig. 14. — The protoplasmic area shown in Fig. 13, more highly magnified, 

 to show the absence of cell outlines. Oc. 2, obj. D. 



Figs. 15 a — d. — Four transverse sections through the youngest ovum, in 

 which the embryo is definitely formed. Ec. Ectoderm, p. Cavity in the 

 proliferating mass of nuclei, p. n. Proliferating mass of nuclei, p. y. Peri- 

 pheral layer of yolk. V. m. Vitelline membrane. T. Yolk within the embryo. 

 Oc. 4, obj. A. 



Fig. 15 a. Section through the anterior end of the ovum, in front of the 

 embryonic region, showing the segmented condition of the peripheral 

 yolk in this region. 

 Fig. 155. Section through the anterior part of the embryonic region, 



showing the embryo surrounded by the peripheral yolk layer. 

 Fig. 15 c. Section through the middle of the embryonic region, showing 

 the embryo surrounded by the peripheral yolk and enclosed in the 

 ectoderm, on one point of which is the proliferating mass of nuclei. 

 The small round bodies are shaded very dark. 

 Fig. 15d. Section through the posterior end of the embryo, shortly an- 

 terior to its termination and behind the region of the proliferating 

 ridge. 

 Fig. 16. — Shows a group of the small round bodies of the peripheral yolk 

 layer from the embryo shown in Figs. 18a — c, highly magnified. They are 

 vacuolated. Reichert's ^ oil immersion. 



Figs. 17 a — c. — Three sections through an embryo, somewhat older than that 

 from which Figs. 15 a — d were drawn. Ec. Ectoderm. Mes. Mesoblast. 

 p. 0. 1. Prseoral lobes, post. Em. Posterior tip of the embryo. Sep. Septum. 

 V. m. Vitelline membrane, Oc. 4, obj. A. 



