THE EMBRYONIC AREA. 



25 



pole of the future embryo. In consequence of the proliferation of the ectoblastic 

 cells, the embryonic area becomes differentiated into a central field, the embryoyiic 

 shield, and a peripheral zone, the area pellucida, which by transmitted light appear 

 respectively dark and light, owing to the varying transparency of the thicker cen- 

 tral and thinner peripheral portions of the germinal field. 



Fig. 22. 



Fig. 



23- 



/-i^P^"'" 



■■-s^; 



Area pellucida 



-Embryonic shield 



, —Embryonic shield 



/ 



-Area pellucida 



^ — Wall of blastodermic 

 vesicle. 



-i — Wall of blastoder- 

 1 mic vesicle 



Embryonic area of rabbit of about seven days, seen from 

 the surface. X 26. {Kallmann.) 



Fig. 24. 



V~ 



\ 



'^- 



1 



Embryonic area of rabbit of about six and one- 

 half days, seen from the surface by transmitted light. 

 X 26. (Kollmann.) 



Coincidently with the assumption of the oval or pyriform outline, a linear thick- 

 ening of ectoderm appears towards the smaller end of the embryonic area. This is 

 X.\\e primitive streak, which grows backward from a terminal thickening, the 7tode of 

 Hensen, that marks its anterior extremity. The primitive streak indicates the direc- 

 tion of the longitudinal axis of the future em- 

 bryo and is modelled by a shallow furrow, 

 the primitive groove, due to the proliferation 

 of the surrounding ectoderm. From the 

 sides of the primitive streak cells are budded 

 off to form the mesoderm, which grows 

 between the outer and inner germ-layers 

 until it, finally, surrounds the blastodermic 

 vesicle. At first, the mesoblast extends 

 laterally and posteriorly iand, later, grows 

 forward as two lateral wings, that embrace 

 the head-end of the embryonic area. While 

 for a time attached only to the ectoderm, 

 the primitive streak subsequently fuses with 

 the entoderm, so that sections across the 

 streak show all the germ-layers blended. 

 The primitive streak is a transient organ and 

 later entirely disappears ; it contributes, 



however, the rapidly growing mesoblastic tissue, which later becomes related to the 

 anal region and the tail-bud. 



The Significance of the Primitive Streak and the mode of formation of the 

 mesoblast are vexed problems in embryology. A brief note on this topic will suffice 

 here. In amphioxus, the lowest vertebrate, the immediate result of segmentation is 

 a hollow sphere, theblastu/a, filled with fluid, lined by a single layer of cells. Invagi- 

 nation at one point of the wall of the blastula occurs, forming eventually a two-layered 

 cup, the gastrula, the outer layer of which is the ectoblast, and the inner one the ento- 

 blast. The cavity within the entoblast is the archenteron or primitive gut. The open- 

 ing into the archenteron is the blastopore. Cells given off from the entoblast, near the 

 blastopore, form a third layer, the mesoblast. Typical gastrulation does not occur in 

 the higher animals, although in the early human embryo a canal appears, known as 

 the neurenteric canal, the opening of which is often regarded as homologous with 

 the blastopore. The primitive streak is regarded by some authorities, notably Hert- 

 wig, as an elongated blastopore with lips fused. Since the primitive streak, the prod- 

 uct of the outer germ-layer, is the principal primary source of the mesoblast, the 

 latter may be regarded as indirectly derived from the ectoblast. A limited secondary 

 and later production of mesoblast is attributed by some to the inner germ-layer. 



Head process 



-Hensen's node 



-Primitive streak 



-Embryonic area 



Extra-embryonic 

 part of blasto- 

 dermic vesicle 



Embryonic area of rabbit of about eight days, seen 

 from the surface. X 20. {After Van Beneden.) 



