36 
GEORGE BROOK. 
EXPLANATION OF PLATE III, 
Illustrating Mr. Brook’s paper “ On the Origin of the Hypo- 
blast in Pelagic Teleostean Ova.” 
y. Yolk. U. Blastodisc (archiblast). ep. Epidermic layer of the epiblast. 
p. Periblast, s. c. Segmentation cavity, v. Vacuoles in the yolk. r. Thickened 
rim of the blastoderm. r v Part of thickened rim included in the embryonic 
shield, c. Nuclei and cells in the yolk. e v Nuclei and cells loose on floor of 
segmentation cavity. c 2 . Nuclei and cells in periblast (margin). c 3 . Nuclei 
and cells in periblast (centre). 
Pig. 1. — Transverse section of the blastodisc of Trachinus vipera taken 
at 7 a.m., and about twenty hours earlier than Fig. 2. X 70. 
Fig. 2. — Section of Trachinus egg, shortly before commencement of hypo- 
blast. x 120. 
Fig. 3. — Section of egg, one and a half hours later than Fig. 2. X 130. 
Fig. 4. — Section of egg, one and a half hours later than Fig. 3. x 130. 
Fig. 5. — Section of egg, half an hour later than Fig. 4. X 130. 
Fig. 6. — Section of egg, half an hour later than Fig. 5. X 130. 
Fig. 7. — Optical section of a living egg of Motella mustela, showing 
segregation of hypoblast well advanced, x 100. 
