238 MR GEORGE BROOK ON THE 



Fig. 12. — Section of egg, twenty-six hours after fertilisation. Morula mass of cells (primary morula). 

 An outer row of flattened epithelioid cells is differentiated = epidermal layer of epiblast of 

 authors. Remaining mass consists of rounded cells loosely aggregated = nervous layer of 

 epiblast of authors. Cortical protoplasm has left yolk pole to collect under and around 

 segmented disc and nuclei, and outlines of cells can he made out in the subgerminal para- 

 blast. Primary segmentation has ended, and secondary segmentation has begun. Section 

 is not quite through centre of egg. Zeiss, A. A. 



Plate XIV. 



Fig. 13. — An imperfect section of a stage shortly before that of fig. 12. Parablast is collecting towards 

 disc. Zeiss, A A. 



Fig. 14. — Egg, twenty -eight hours after fertilisation. Section through marginal part of morula (haema- 

 toxylin). Cells from archiblast easily distinguisbed from those which have been recently 

 added from the parablast. Archiblast cells are stained more deeply, and there are 

 indications of intracellular reticulum. Numerous vacuoles are seen between archiblast 

 cells and bridges of protoplasm connecting the neighbouring cells together. Zeiss, D D. 



Fig. 15. — Section of an egg, thirty hours after fertilisation. Unsegmented portion of parablast is seen 

 as a thin film beneath the segmented disc. End of first budding-off process in parablast. 

 Swift, 1 in. 



Fig. 16. — Margin of morula of a stage about same as fig. 15, more highly magnified to compare 

 with fig. 14. All distinction between the two kinds of cells is lost (lueniatoxylin). 

 Zeiss, D. D. 



Fig. 17. — Egg, thirty-four hours after fertilisation. Section represents basal portion of disc with the 

 adjoining parablast ; the two portions are not in contact at periphery, but this is the result 

 of a mechanical injury. Cells are again to be seen in process of formation in the parablast. 

 Differential carmine staining shows gradual transition from cells in process of formation at 

 the base of the parablast, to those already included in the germinal disc. Swift, A 



Fig. 18. — Section of egg, forty-five hours after fertilisation. The final morula, containing in herring 

 parablastic as well as archiblastic elements. Subgerminal portion of parablast is only a 

 thin layer, peripheral portion, however, forms a thick wedge-shaped mass, extending from 

 base of morula to equator of egg, and contains a considerable number of nuclei. Swift, 1 in. 



Fig. 19. — Right corner of morula of fig. 18, more highly magnified and showing the nuclei in the 

 peripheral parablast. Swift, A. 



Fig. 20. — Section of egg, fifty-six hours after fertilisation, showing beginning formation of segmentation- 

 cavity. Parablast forms its floor, and the cells of the morula form its roof and lateral 

 boundaries. Nuclei are seen to be distributed throughout the parablast. Swift, 1 in. 



PlATE XV. 



Fig. 21. — Longitudinal section of disc 'n axis of embryo, sixty-eight hours after fertilisation. Shows 

 roof of segmentation-cavity formed of several rows of cells, its floor of parablast. Thicken- 

 ing at periphery forms commencement of blastodermic rim, and the layer pushed inwards 

 from this thickening constitutes primitive hypoblast. Swift, 1 in. 



Fig. 22. — More magnified view of a portion of fig. 21. Absence of nuclei in thinned out portion of 

 parablast immediately below rim, but they are still numerous in peripheral parablast, and in 

 parablast forming door of segmentation-cavity. In region of blastodermic rim the cells of 

 the morula rest directly on the parablast ; but centrally the two layers are separated by 



