THE PRIMITIVE STREAK OP THE RABBIT. 219 



EXPLANATION OF PLATES 20—22, 



Illustrating Mr. Richard Assheton's paper on " The Primitive 

 Streak of the Rabbit ; the Causes which may determine 

 its Shape, and the part of the Embryo formed by its 

 activity." 



List of Reference Letters. 



A, Anterior end. AM. Amniotic cavity. A. PS. Anterior end of primi- 

 tive streak, END. H. Endotlielial lining of heart. EP. Epiblast. HN. 

 Henseu's node. HT. Hypoblast. M. Centre of primary area of cell-pro- 

 duction. MES. Primitive streak mesoblast. MES. BY. Hypoblastic meso- 

 blast. N. Centre of secondary area of cell-production. NCH. Notochord. 

 NQ. Neural groove. NP. Neural plate. P. Posterior end. PC. Peri- 

 cardial band. P. GR. Primitive groove. P. PS. Posterior end of primitive 

 streak. PS. Primitive streak. Z. Point V7hich marks approximately the 

 boundary between the results of the primary centre of growth and that of 

 the secondary centre of growth. 



PLATE 20. 



Fig, 1. — Surface view of the embryonic disc of a rabbit embryo of the 

 150th hour, x 18. 



Fig. 2. — Surface view of the embryonic disc of a rabbit embryo, at the 

 earliest moment at which the primitive streak activity is perceptible as a 

 thickening. Age about 158 hours, x 18. 



Fig, 3. — Surface view of the embryonal area of a rabbit embryo, in which 

 the primitive streak is very evident. Age about 168 hours, x 18, 



Fig. 4. — Surface view of the embryonal area of a rabbit embryo, in which 

 the process of lengthening of the primitive streak is well advanced. Age 

 about 168 hours, x 18. 



Fig. 4 a. — Surface view of the embryonal area of a rabbit embryo, slightly 

 more advanced than the preceding. Age about 168 hours, x 18. 



Fig, 5. — Surface view of the embryonal area of a rabbit embryo, in which 

 the primitive streak has become greatly elongated and is grooved slightly. 

 Age about 172 hours, x 18, 



Fig. 6. — Surface view of the embryonal area of a rabbit embryo, in which 

 the primitive streak has attained its maximum length, and is most deeply 

 grooved. This figure has unfortunately been drawn slightly larger than the 

 photograph from which it was copied, on which the measurements in the text 

 were mad«. The error is about ^. Age about 180 hours, X 18, 



