226 BIOLOGY: GENERAL AND MEDICAL 



where the spinal cord will form, and the folds on each 

 side of the groove, two arches of the ectoderm by con- 

 crescence or fusion of which the spinal canal will event- 

 ually be formed. 



While these wrinkles or folds are preparing the way 

 for future development, the mesoderm, from which the 

 skeletal, motor, circulatory, and connective tissues are to 

 form, is being prepared by the penetration into the space 

 between the ectoderm and entoderm, of a mass of small 

 cells in many superimposed layers, arising from both 

 ectoderm and entoderm, and as these cells become in- 



IK i/C 



FIG. 89. Cross-section through the middle of the primitive streak of a chick's 

 germ disc. At some distance from the primitive groove is to be seen upon the 

 left side of the figure in cross-section the marginal groove of His; upon the right 

 side it is as yet little developed, ak, Outer, ik, inner, mk, middle germ layers; 

 pr, primitive groove; ps, primitive streak; gr, marginal groove. (After Roller.) 



closed between the two chief blastodermic layers they 

 also become separated from the parent blastodermic 

 layers and differentiated as a third blastodermic layer. 



A transverse section of the germinal disc at the time 

 of the formation of the mesoderm, which coincides fairly 

 well with the period of the formation of the primitive 

 streak and groove, is shown in the illustration. 



The future development of such meroblastic eggs 

 depends upon their character. If the eggs are enclosed 

 in shells, development is complicated by the formation 

 of a membrane investing the embryo the amnion; if 

 the eggs are without shells, as those of fishes, there are 

 no membranes, and development is less complicated. 

 In both cases the growth of the embryo continues by 

 folding, convergence, and concrescence of the cellular 

 germinal plate, more and more completely separating 

 the embryo m>m its yolk. 



Now, leaving the meroblastic eggs of the fishes, rep- 



