hEVELOVMKXr OF THE FaUCS. 97 



and the dorsal cord ; the upper ring represents the rudiments of tlie 

 annular portion of the vertebrir, while the lower ring and the dorsal 

 cord constitute the vertebral bodies and the discs uniting them. 



The lateral lamince arise from the portion of tlie middle layer of the 

 blastoderm placed on each side of the vertebral laminte. In the region 

 of the trunk tliese lamiuic are separated for a certain time fi-om the 

 latter; but in the cephalic region they are always adherent to them, 

 and at this part they are usually designated the cephalic lam'nue. The 

 'proper lateral lamime are divisible into two layers, external and internal, 

 united by a viiddlc layer ; they comprise between them a space which 

 becomes the plcuro-peritoneal cavitij, after the formation of which the 

 lateral are joined to the vertebral lamina. The internal or fihro-intes- 

 iinal layer envelops the deeper portion of the blastodermic layer or 

 intestinal furrow, the umbilical vesicle, and the allantois ; it constitutes 

 the tibrous and vascular parts of these membranes, and carries the ves- 

 sels to the inner face of the chorion. The external or cutaneous layer 

 is developed in two ways : above, it glides between the muscular laminie 

 and the fa^tal portion of the external layer of the blastoderm to form 

 the cutaneous envelope on the back ; below, it separates into two leaves 

 which receive between them the prolongations of the muscular lamiucC 

 destined to constitute the intercostal, abdominal, and other muscles of 

 this part of the body. Of these two secondary leaves, the external 

 forms the skin of the trunk, and the internal the parietal layer of the 

 peritoneum. The cutaneous lamin® also furnish an extra-foetal pro- 

 longation — the fibrous layer of the amnion. 



The middle or mesenteric lamime join at the median line, and in their 

 substance are developed the Wolttian bodies, or antecedent deciduous 

 kidneys, and the principal vessels of the trunk. 



The cephalic lamina:' always remain adherent to the vertebral laminae, 

 and are inflected inwards with them to form the anterior part of the 

 cephalo-intestinal cavity, which is divided into two compartments — the 

 pharyngeal and a'sophageal cavities. Tlie pharyngeal cavity opens 

 externally by the mouth, and is partly enclosed on the sides by the 

 pharyngeal arches. The a-sophageal cavity soon shows a diverticulum, 

 which is not long in communicating with the pleuro-peritoneal cavity, 

 and subsequently contains the heart ; it is therefore named the cardiac 

 cavity. The cephalic lamina} also form the derm of the cranium, and 

 the fibrous layer in which are developed some of the cranial boues. 



The Nervous System. 



The development of the nervous system comprises the growth of the 

 brain, spinal cord, and nerves. The initial steps in the development of 

 the brain and cord have been already indicated. At each extremity of 

 the medullary cavity, which is a modification of the median furrow, is a 

 slight bulging. From the posterior, or rhomboidal sinus, the sacro- 

 lumbar nerves are given off, while the anterior gives origin to the brain. 

 This anterior enlargement appears as tlirce successive dilatations named 

 the cerebral vesicles or cells, which are distinguished as anterior, middle, 

 and posterior. They are filled witli fluid, and the middle slightly sur- 

 mounts the other two, which gives the whole the figure of a small 

 triangular mass. The vesicles increase irregularly in volume, and their 

 walls, in developing, form the nervous tissue ; while their cavity persists 

 and becomes the space observed in each portion of the encephalon. The 

 anterior vesicle represents the cerebral hemispheres, the thalami optici 



7 



