STRUCTURES FORMED FROM THE EPIBLAST. 



865 



Fig. 664. 

 Vertical section of part of the unincubated blastoderm of a hen. 

 a, epiblast ; b, hypoblast ; c, formative cells resting on white 

 yelk ; /, archenteron. 



the intestinal epithelium, and that of the glands which open into intestine. The 

 notochord is also formed from its axial portion. [The mouth and anus being 

 formed by an inpushing of the epiblast, are lined by epiblast, and are sometimes 

 called the stomodaeum and proctodeum respectively.] 



[Structure of the Blastoderm (fig. 664). Originally it is composed of only two 

 layers, and in a vertical section of it the epiblast consists of a single row of 

 nucleated granular cells, t, 



arranged side by side, 

 with their long axes placed 

 vertically. The hypoblast 

 consists of larger cells 

 than the foregoing, al- 

 though they vary in size. 

 They are spherical and 

 very granular, so that no 

 nucleus is visible in them. 

 The cells form a kind of 

 network, and occur in 

 more than one layer, espe- 

 cially at the periphery. It rests on white yelk, and under it are large spherical 

 refractive cells, spoken of as formative cells (c).] 



The cells of the epiblast, and especially those of the hypoblast, nourish themselves by the 

 direct absorption and incorporation of the constituents of the yelk into themselves. The 

 amceboid movements of these cells play a part in the process of absorption. The absorbed 

 particles are changed, or, as "it were, digested within the cells, and the product used in the 

 processes of growth and development (Kollmann). 



440. STRUCTURES FORMED FROM THE EPIBLAST. Laminae Dor- 

 sales. The medullary groove upon the epiblast (also called outer, serous, sensorial, 

 corneal, or animal layer) becomes deeper (fig. 665, II). The two longitudinal 

 elevations or laminae dorsales consist of a thickening of the epiblast, and grow up 

 over the medullary groove, to meet each other and coalesce by their free edges in 

 the middle line posteriorly. Thus, the open groove is changed into a closed tube 

 the medullary or neural tube (III). The cells next the lumen of the tube 

 ultimately become the ciliated epithelium lining the central canal of the spinal 

 cord, while the other cells of the nipped-off portion of the epiblast form the 

 ganglionic part of the central nervous system and its processes. 



Primary Cerebral Vesicles. [The laminae dorsales unite first in the region of the 

 neck of the embryo, and soon this is followed by the union of those over the future 

 head.] The medullary tube is not of uniform diameter, for at the anterior end it 

 becomes dilated and mapped out by constrictions into the primary vesicles of the 

 brain, which at first are arranged, one behind the other, in the following order, 

 each one being smaller than the one in front of it : the fore-brain (representing 

 the structures from which the cerebral hemispheres are developed) ; the mid-brain 

 (corpora quadrigemina) ; the hind-brain (cerebellum) ; and the after-brain 

 (medulla oblongata), which is gradually continued into the spinal cord (IV and V). 

 The posterior part of the medullary tube has a dilatation at the lumbar enlarge- 

 ment. In birds, the medullary groove remains open in this situation to form a 

 lozenge-shaped dilatation, the sinus rhomboidalis. 



Cranial Flexures. The anterior part of the medullary tube curves on itself, 

 especially at the junction of the spinal cord and oblongata, between the mid-brain 

 and hind-brain, and again almost at right angles between the fore-brain and mid- 

 brain. [Thus, a displacement of the primary vesicles is produced, and the head of 

 the future embryo is mapped off.] At first all the cerebral vesicles are devoid of 

 convolutions and sulci. On each side of the fore-brain there grows out a stalked 



