58 TEXT-BOOK OF EMBRYOLOGY. 



quently many of the cells that form the roof of the archenteron are not 

 brought in by the invagination but by involution. 



There is still another factor in gastrulation. It has already been 

 noted that on surface view the groove moves downward as the highly pig- 

 men ted cells along its upper or dorsal lip encroach upon the non-pigmented 

 area, so that when the groove becomes ring-shaped only a small yolk area is 

 visible. This downward growth over the yolk area, or epiboly, which is 

 more rapid on the side where the groove began, results in the enclosure of 

 more and more yolk cells so that only those comprising the yolk plug are 

 left exposed. It is this process (epiboly) therefore which causes the 

 lessening of the crescent and ring as seen on surface view. (Compare Fig. 



30.) 



These processes which are grouped under the term gastrulation have 

 converted the single-layered blastula into the double-layered gastrula. The 

 outer layer composed of several strata of pigmented cells is the ectoderm which 

 is in contact with the environment. The inner is the entoderm which lines 

 the archenteric cavity. Two types of entodermal cells are distinguishable: 

 those forming the roof and sides of the archenteron which contain a moderate 

 amount of pigment and those forming the floor which hold little pigment but 

 an abundance of yolk. The two primary germ layers are continuous at the 

 rim of the blastopore. 



Two other features which are incidental to the processes of gastrulation 

 must be noted because of their bearing upon future development. Recalling 

 the migration of the crescentic groove, which eventually becomes the ring 

 around the yolk plug, it is obvious from the manner in which the migration 

 occurs that the cells along the horns of the crescent are drawn toward the 

 median region. The name given to this phenomenon is concrescence. The 

 result of it is that the cells are piled up in a median linear strand, from which 

 the rudiments of certain organs emerge. The outer feature is the 

 flattening of the ring from side to side, concomitant with the withdrawal 

 /inward and disappearance of the yolk plug, so that the two lateral margins 

 approximate, leaving only a narrow slit leading from the exterior into the 

 archenteric cavity. Subsequently the slit is closed by fusion of its walls, 

 but part of the depression in its site becomes the anal pit or proctodaeum. 



At this stage the gastrula is still spherical and only slightly larger than 

 the blastula. It possesses the same fundamental arrangement of structure 

 as the gastrula of Amphioxus. The ectoderm forms contact with the envir- 

 onment, implying response to stimuli and protection; and the organs corre- 

 lated with these functions are derived from this layer. The archenteric 

 cavity with its lining of endoderm is confined to the interior of the developing 

 organism and comprises the primitive alimentary system. Within the 



