FORMATION OF THE GERMINAL LAYERS IN TELEOSTEI. 239 



the segmentation-cavity. Hence the close union of the primitive layers at periphery of 

 disc. Swift, i. 



Fig. 23. — Transverse section through axis of embryo, 77£ hours after fertilisation. Shows primitive 

 hypoblast in close union with parablast, from which it is derived, but almost completely 

 separated from the cells forming roof of segmentation-cavity. Swift, 1 in. 



Fig. 24. — More highly magnified portion of fig. 23. Shows the intimate connection between yolk and 

 parablast. Upper part of parablast contains a number of free nuclei, agreeing in every 

 particular with the nuclei of the primitive hypoblast cells, and around some of them proto- 

 plasm is constricted off to form cells. Zeiss, E. 



Fig. 25 — Longitudinal section, in axis of embryo of an egg, 92| hours after fertilisation. Blastopore 

 is near its closure. Primitive hypoblast appears to cease at posterior portion of head 

 swelling, due to its being in this region pushed to each side by the enormous development 

 of rudiment (keel) of central nervous system. Nuclei are still prominent in the parablast, 

 and the two primitive layers are separated by a small space in neck region, but are united 

 towards caudal extremity. Swift, 1 in. 



Fig. 26. — Shows a portion of fig. 25, in region of tail swelling, more highly magnified. Cells in epiblast 

 are closely packed together, whereas those of primitive hypoblast are more loosely arranged. 

 The two layers are, however, not distinctly separated, though a difference in the staining of 

 the nuclei in the original preparation indicates point of union of the two layers. Parablast 

 shows as a thin unsegmented layer of protoplasm, in which are embedded numerous nuclei. 

 Zeiss, D. I). 



Fig. 27. — Longitudinal section in axis of embryo, 101 J hours after fertilisation. Blastopore is just 

 closed. Nervous epiblast is conspicuous in anterior region. Mesoblast is beginning to 

 show division into somites, and hypoblast is distinctly separated in region of somites. 

 Zeiss, A A. 



Fig. 28. — More highly magnified portion of same embryo, as fig. 27, showing epiblast, mesoblast, and 

 hypoblast, the last still in connection with the unsegmented protoplasm of the parablast. 

 Zeiss, D D. 



Fig. 29. — Transverse section through body axis of an embryo of same stage as figure 27. Upper part of 

 section passes through the head region, and shows the thickened nervous epiblast (keel). 

 Lower part passes through tail region ; notochord not yet differentiated. Zeiss, A A. 



Fig. 30. — Transverse section of an embryo, 112 hours after fertilisation. Shows separation of the 

 notochord and the two lateral mesoblastic plates. A single row of cells remains in con- 

 nection with the parablast, and constitutes the rudiment of the permanent hypoblast. 

 Zeiss, A A. 



VOL. XXXIII. PART I. 2 K 



