446 Animal Biology 



cavities in which tissues and organs could be placed. Consequently, at a 

 certain stage in the cleavage process, the cells of the morula stage all line 

 up in a very definite fashion to form a one-layered, hollow sphere known 

 as the hlastula or hollow sphere stage (Fig. 220, D, E, F) . This sphere 

 has a central, fluid-filled cavity known as the blastocoel or segmentation 

 cavity. This hlastula stage consists of ( 1 ) an outer, transparent, jellylike 

 capsule; (2) a dark, pigmented animal hemisphere, composed of smaller 

 and more numerous cells (ectoderm) ; and (3) a light-colored, unpig- 

 mented vegetal hemisphere which floats downward and is composed of 

 larger and fewer cells (entoderm). The vegetal cells are quite large and 

 contain yolk or food which is supplied to the cells of the animal hemi- 

 sphere. This arrangement makes the wall of the hlastula on the vegeta- 

 tive side much thicker than it is on the upper or animal side. The active 

 growth in this stage occurs, primarily, in the animal hemisphere region. 



The gastrula or yolk plug stage follows the hlastula and is formed as 

 follows : At a certain point between the animal and vegetal hemispheres, 

 the vegetal cells turn inwardly into the blastocoel or segmentation cavity 

 (Fig. 220, G, H). The pigmented animal cells (ectoderm) grow over 

 the lighter colored, unpigmented vegetal cells (entoderm) and fold in 

 with them to some extent at that point. Thus, an inner area or layer 

 of cells is continuous with the outer layer. Because of more rapid mitosis, 

 the animal hemisphere continues to grow almost entirely over the vegetal, 

 leaving a small, light yolk plug exposed. The space between the bound- 

 aries of the infolded layers of cells, which surrounds the yolk plug, is 

 the blastopore or primitive mouth. The ingrowth of the latter is shown 

 on the surface by a thin, crescent-shaped fold or groove. The outer 

 layer of cells is known as the ectoderm and is continuous with the inner 

 inturned layer or entoderm. The point where the entoderm cells of the 

 vegetal region turn in is one side of the yolk plug and is known as the 

 dorsal lip of the blastopore. This inturned entoderm forms a cavity 

 known as the archenteron or primitive intestine (primitive gut). The 

 blastocoel now appears as a reduced cavity at the opposite side and is 

 gradually being crowded out by the developing archenteron and the 

 entoderm. The indentation on the opposite side of the yolk plug from 

 the dorsal lip of the blastopore is known as the ventral lip of the blasto- 

 pore. 



The neural groove stage (Fig. 220, 7) follows the gastrula stage. The 

 neural groove, which is the forerunner of the future nervous system, be- 

 gins as a small depression on the dorsal side of the blastopore and grows 

 anteriorly along the dorsal side of the embryo as a thickened neural plate 



