32 



CLEAVAGE AND THE GERM LAYERS 



P^^^^ 



Area opaca 



Primitive knoi 

 Primitive pit 



Primitive fold 



Primitive groove 



Area pellucida 

 Blood island 



Fig. 23. — Blastoderm of a chicfc embryo 

 at the stage of the primitive streak and 

 groove (16 hours). X 20. 



fixed while the edges of the margin on each side are carried caudad and brought 

 together. Thus a crescentic margin is transformed into a longitudinal sKt as in 

 Fig. 24. Since this marginal Hp originally bounded the blastopore (p. 29) the 

 longitudinal sUt must also be an elongated blastopore whose direction has merely 



been changed. The lips of the sHt fuse, 

 forming the primitive streak. The primi- 

 tive groove may be interpreted as a further 

 futile attempt at invagination in the region 

 of the blastopore. The teachings of com- 

 parative embryology support these con- 

 clusions, for the neurenteric canal arises 

 at the cranial end of the primitive streak, 

 the anus at its caudal end, while the 

 primary germ layers fuse in its substance. 

 All these relations exist at the blastopore 

 of the lower animals. 



From the thickened ectoderm of the 

 primitive streak a proHferation of cells 

 takes place and there grows out laterally and caudally between the ectoderm and 

 entoderm a sohd plate of mesoderm (Fig. 3i B and C). From the primitive knot 

 a mesodermal sheet al soe xtends cephal a d forming along th e_inidlin&-ar-tbi^er 

 laver, the _so-called head p rocess or notochordal plate , which fuses intimately with 

 the, entoderm (Figs. 25, 30 and 31 A). 

 Since the primitive streak and groove 

 represent a modified blastopore, it is 

 evident that this cranial extension, the 

 head process, corresponds to the pouch- 

 like invagination concerned in the forma- 

 tion of mesoderm and notochord in rep- 

 tiles. In birds the fusion of the head 

 process with the entoderm, the relation 



of mesodermal sheets to it laterally, the ^^''^'^ =^"5^^ f™°> it by the fusion of the edges 



. of the crescent, 



tormation of the notochord from its tis- 

 sue, and the occasional traces in it of a cavity continuous with the primitive pit 

 (i. e., neurenteric canal), all recall the conditions described for the less modified 

 invagination in reptiles. 



Mammals. — On the blastoderm of mammals appear a primitive streak and 



ff" 







/#% 



' 1 If \ \ \ \ 



1 1 : 1 1 1 1 1 



Fig. 24. — Diagram elucidating the forma- 

 tion of the primitive streak (Duval in Heisler). 

 The increasing size of the germ disc in the 

 course of the development is indicated by dot- 

 ted circular lines. The hea\'y lines represent 

 the crescentic groove and the primitive streak 



