THE FOKMATION OF THE EMBKYO. 37 



cavity of the primitive mid-brain is converted into the aqueductus cerebri, which 

 connects the third with' the fourth ventricle. 



After the anterior and posterior neuropores (p. 31) are closed, the cavity of 

 the neural tube is, for a time, a completely enclosed space. Subsequently the 

 mesoderm, which in the meantime has surrounded the tube, becomes differentiated, 

 in its immediate neighbourhood, into three membranes. The innermost of the 

 three is closely connected with the walls of the neural tube and is called the 

 pia mater. The outermost, known as the dura mater, is dense and resistant, and 

 the intermediate membrane is a thin lamella called the arachnoid. 



As the membranes are formed, spaces are differentiated between them. The 

 space between the dura mater and the arachnoid is the subdural space, and that 

 between the arachnoid and the pia mater is the subarachnoid space. 



After a time a median perforation, the median aperture of the fourth ventricle 

 (O.T. foramen of Magendie), and two lateral perforations pierce the dorsal wall 

 of the fourth ventricle and the pia mater which covers it, and thus the fourth 

 ventricle becomes connected with the subarachnoid space. It is stated also that 

 a perforation passes through the medial wall and the covering pia mater of a 

 portion of each lateral ventricle which is called its inferior horn, throwing those 

 portions of the lateral ventricles also into communication with the subarachnoid 

 space, but it is doubtful if the statement is correct. 



THE FOKMATION OF THE EMBEYO. 



Mesoderm of amnion, 

 Primitive streak 



Body stalk 



Allantoic 

 'diverticulum 

 from entoderm 

 vesicle 



Notochord 



The transformation of the relatively flat embryonic area into the form of the 

 embryo is due, in the first instance, to the rapid extension of the median part 

 of the area, as contrasted with 

 the slower growth of its mar- 

 gins, and the later modelling 

 of the various parts of the 

 embryo is due to different rates 

 of growth in different parts of 

 the embryonic region. 



By the rapid proliferation 

 of cells from the nodal grow- 

 ing point, at the cephalic end 



nf fV,a T* ' V a at V fk FlG ' 48 ' SCHEMA OF SAGITTAL SECTION OF EMBRYONIC AREA AND 



eaK, tne AMNION BEFORE THE FOLDING OF THE AREA HAS COMMENCED. 

 cephalo-caudal length of the 

 area is increased, whilst 

 the cephalic and caudal 

 ends of the area remain 

 relatively fixed, conse- 

 quently the area be- Region of 

 comes folded longitu- ne a u ? e p r or f" 

 dinally. At the same 

 time, the cephalic end 



Of the neural groove is Buoco-pharyngeal/ / 



, , f? , -, membrane i Pericardium 



pushed away from the 



Amnion^cavity 

 Neural tube\ 



Ectoderm of amnion 



Amniotic mesoderm 

 Chorionic 

 mesoderm 



Region of 

 posterior 



neuropore 



Cloacal 

 - membrane 

 Body stalk 



^ Allantoic 

 diverticulum 



Hind -gut 



Mid-gut 

 Fore-gut (heart not shown) 



nodal point, until it lies 

 at first dorsal and then 

 cephalad to the cephalic 

 border of the area. As 

 a result of this move- 

 ment the bucco-pharyn- 

 geal and the pericardial areas become reversed in position, and a cephalic or head 

 fold is formed. This fold is bounded, dorsally, by what is now the cephalic portion 

 of the embryo, ventrally, by the reversed pericardial region, and its cephalic 

 end is formed by the extremity of the head region and the bucco-pharyngeal 

 membrane. 



FIG. 49. SCHEMA OF SAGITTAL SECTION OF EMBRYONIC AREA SHORTLY 

 AFTER THE FOLDING HAS COMMENCED. The pericardial mesoderm is 

 carried into the ventral wall of the fore-gut and the coelom has extended 

 through it. The cephalic end of the neural tube and the caudal pait of 

 the primitive streak are bent ventrally, and the latter now forms the 

 cloacal membrane. 



