24 



INTRODUCTION 



The dorsal wall of the archenteron consists of relatively few layers 

 of small endoderm cells, while the floor consists of many layers of 

 large yolk-laden cells. Certain cells along the dorsal mid-line 

 differentiate into a rod-like structure, the notochord, and other 

 cells proliferate off on either side of the notochordal region to form 

 mesoderm masses separated from ectoderm and endoderm. Eventu- 

 ally the right and left sheets meet along the mid-ventral line and fuse. 



Before this fusion occurs, the mesoderm mass begins to split 

 into two sheets on each side a short distance away from the noto- 

 chord. This continues so that eventually, as in Amphioxus, there 

 will be an outer somatic sheet associated with ectoderm and an 

 inner splanchnic sheet associated with endoderm. The space 

 between them is the coelome, or body cavity. 



Blastoderm 



Segmentation 

 cavity 



Ectoderm Blastocwle 



A nterior 



Archenteron 



Invaginated 



endoderm 



Blastopore 



Neural fold 

 Neural groove 

 Mesodermal somite 

 ~Somafopleure 

 ~S plan ch nopleu re 

 Endoderm- 



Fig. 4. — Diagram of early stages in development of reptiles and birds. (Modified 

 from Arey's Developmental Anatomy, W. B. Saunders Company.) 



Ectoderm- 

 Somatic mesoderm-,;^ 

 Splancfinic mesoderm '^ 



Sauropsida. — In reptiles and birds, where the egg contains a large 

 amount of yolk, the nucleus with a small amount of cytoplasm is 

 located at the animal pole. This restricted protoplasmic region 

 divides into cells, but the larger yolk portion takes no part in 

 cleavage. Cleavage is therefore partial and results in a small 

 disc of cells on top of the spherical yolk mass. This disc of cells, 

 called a blastoderm, continues to divide by a series of vertical and 

 horizontal planes, so that in the late cleavage stage there are a few 

 layers in the central ])art of the disc, but only a single lay(>r of cells 



