36 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



blastopore and nearly occludes the opening. Make a diagram of the section, 

 coloring ectoderm blue and entoderm yellow. 



3. In meroblastic eggs. In such eggs the formation of the entoderm will 

 obviously be attended by difficulties. It seems to be accomplished in two ways: 

 (a) by a process of involution or imagination, in which the cells at the future 

 posterior end of the blastoderm turn under and grow forward; (b) by a process 

 of denomination, in which the cells on the under side of the blastoderm arrange 

 themselves to form a lower layer. In different forms the proportion of entoderm, 

 formed by each method varies. Eventually the two kinds of entoderm unite 

 to form a single sheet of cells. The cavity of the archenteron is very small in 

 meroblastic eggs, consisting of a cleft between the entoderm and the yolk, and 

 the blastopore is reduced to a slit at the posterior end of the blastoderm. After 

 gastrulation has occurred the blastoderm consists of two layers, an outer ectoderm 

 and an inner entoderm; it lies on the surface of the yolk and by proliferation at 

 its margins gradually spreads out over the yolk, eventually inclosing it. 



The gastrulation of meroblastic eggs by the invagination process is illustrated 

 diagrammatically in Figure $C. 



D. FORMATION OF THE THIRD GERM LAYER, THE NEURAL TUBE, 

 AND THE NOTOCHORD 



These processes occur practically simultaneously but will be described sepa- 

 rately. 



i. In Amphioxus. After the embryo has attained the gastrula stage it 

 elongates and presents a flattened dorsal surface, a rounded ventral surface, and 

 recognizable anterior and posterior ends (see Fig. 5^3). From the dorsolateral 

 regions of the entoderm, which it will be remembered forms the "inner tube" 

 of the gastrula, hollow pouches begin to grow out in pairs. These pouches 

 are called the coelomic sacs or mesodermal pouches. The walls of the pouches 

 constitute the mesoderm, or third germ layer, which, unlike the ectoderm and 

 entoderm, consists of two walls. The pouches grow laterad and ventrad, filling 

 the space between ectoderm and entoderm. The outer wall of the pouches in 

 contact with the ectoderm is called the somatic or parietal mesoderm ; the inner 

 wall in contact with the entoderm is the splanchnic mesoderm. The cavity of 

 the pouches is the body cavity or coelom. Eventually the anterior and posterior 

 walls of the pouches break down so that those of each side unite to form a tube. 

 Thus, the coelom, originally segmented, comes to consist of a pair of continuous 

 cavities, one on each side of the embryo. 



Meantime, the ectoderm rises up on either side of the median dorsal line 

 as a fold or ridge. The two folds meet above the median dorsal ectoderm, which 

 is thus inclosed and becomes the neural tube, the primordium of the brain and 

 spinal cord. 



