FORMATION OF THE GERMINAL LAYERS IN TELEOSTEI. 237 



EXPLANATION OF PLATES. 



Figures 5, 7, and 8 are taken from drawings by my friend Mr W. L. Caldekwood ; figures 6 and 

 9 were originally drawn from the living egg, and the colours are diagrammatic. All the other figures 

 were drawn for me by Mr J. T. Thompson, M.B., CM. I feel it only just to Mr Thompson to state 

 that much of the delicacy of his original drawings has been unavoidably lost in their reproduction on 

 stone. The figures are taken exclusively from ova of herring. 



Plate XIII. 



Fig 1. — Section of viscous layer and egg-membrane of a ripe unfertilised ovum. Externally is seen the 

 homogeneous viscous layer, then follows the outer layer of zona radiata longitudinally 

 striated, and most internally is the inner layer of zona radiata transversely striated. 

 Gundlach, j^, Oc. 3. 



Fig. 2. — Section of abnormal egg-envelopes, showing viscous layer divided into two strata, and each 

 exhibiting a well-marked transverse striation. Gundlach, ^ F , Oc. 3. 



Fig. 3. — Transverse section of an unfertilised ovum. The carmine-stained protoplasm is seen distri- 

 buted throughout the yolk, and not collected into a definite layer. In its meshes lie the 

 large unstained yolk-spheres, and they occupy the greater portion of the egg. Lying imme- 

 diately beneath egg-membrane are seen smaller yolk granules. Swift, 1 inch. 



Fig. 4. — Germinal vesicle of ovarian ovum. Latest stage observed. Shows the irregular, somewhat 

 quadrangular, outline, the fine protoplasmic threads leaving its margin and going into yolk- 

 mass, and the delicate nuclear reticulum. Gundlach, ^. 



Fig. 5. — Section of an unfertilised ovum after lying two days in sea- water. Differs only from recently- 

 matured ovum in that the protoplasmic network has withdrawn a little from centre of egg, 

 and there is rather more protoplasm at periphery, but no disc-like prominence is to be 

 seen. Zeiss, A A, Oc. 3. 



Fig. 6. — Optical section of a living egg, one hour after fertilisation. Germinal protoplasm is seen 

 collecting at circumference as a continuous layer, considerably thicker at one side. Yolk 

 granules are no longer visible. There is a large breathing-chamber. Zeiss, A A, Oc. 3. 



Fig. 7. — Actual section of an ovum, one hour after fertilisation. Shows true relation of the protoplasm 

 to the yolk. The protoplasm is collecting at surface, but there is still left a portion mixed 

 amongst the yolk-spheres in the shape of branching processes, which towards germinal pole 

 are stronger, and penetrate further into yolk. Zeiss, A A, Oc. 3. 



Fig. 8. — Section of an egg, five hours after fertilisation. Germinal protoplasm has almost entirely 

 collected at germinal pole. Large masses of yolk are entangled in the meshes of its 

 processes, and a number of yolk-masses, varying in size, are also found in body of germinal 

 mound itself. Zeiss, A A ; Oc. 3. 



Fig. 9. — Egg, nine hours after fertilisation ; four-cell stage. Sketched from living egg, and coloured 

 to harmonise with the other figures. Shows the separation of parablast from archiblast 

 by the formation of an equatorial furrow. Lacunae are indicated in the yolk, which are 

 continued towards the base of disc by stalks. Swift, 1 inch. 



.Fig. 10. — Section of egg, twenty-one hours after fertilisation. Three rows of cells are seen in germinal 

 disc. Cell reticulum is well brought out by differential carmine stain. Zeiss, A A. 



Fig. 11. — Section of egg, twenty-four hours after fertilisation, showing morula shortly before addition of 

 cells from parablast. The protoplasm of parablast is chiefly situated at yolk pole at this 

 stage. Zeiss, A A. 



