SEGMENTATION OF THE FERTILIZED OVUM 



49 





sudoriferous^ and mammary glands, the hairs and nails, the epithelium of the nose 

 and adjacent air sinuses, and that of the cheeks and roof of the mouth. From it 

 also are derived the enamel of the teeth, and the anterior lobe of the hypophysis 

 cerebri, the epithelium of the cornea, conjunctiva, and lacrimal glands, and the 

 neuro-epithelium of the sense organs. 



Entoderm. The entoderm consists at first of flattened cells, which subsequently 

 become columnar. It forms the epithelial lining of the whole of the digestive tube 

 excepting part of the mouth and pharynx and the terminal part of the rectum 

 (which are lined by involutions of the ectoderm), the lining cells of all the glands 

 which open into the digestive tube, including those of the liver and pancreas, 



V. 



Fiq. 16. A series of transverse sections through an embryo of the dog. (After Bonnet.) Section I is the most 

 anterior. In V the neural plate is spread out nearly flat. The series shows the uprising of the neural folds to form the 

 neural canal, a. Aortae. c. Intermediate cell mass. ect. Ectoderm, ent. Entoderm. h, h. Rudiments of endothelial 

 heart tubes. T 

 derm are th< 

 plate still undi 

 mes. Mesoderm. p. Pericardium, so. Primitive segment. 



the epithelium of the auditory tube and tympanic cavity, of the trachea, bronchi, 

 and air cells of the lungs, of the urinary bladder and part of the urethra, and that 

 which lines the follicles of the thyroid gland and thymus. 



Mesoderm. The mesoderm consists of loosely arranged branched cells sur- 

 rounded by a considerable amount of intercellular fluid. From it the remaining 

 tissues of the body are developed. The endothelial lining of the heart and blood- 

 vessels and the blood corpuscles are, however, regarded by some as being of ento- 

 dermal origin. 



As the mesoderm develops between the ectoderm and entoderm it is separated 

 into lateral halves by the neural tube and notochord, presently to be described. A 

 4 



