24 



HUMAN EMBEYOLOGY. 



Anterior end of neural fold 

 Plasmodial trophoblast 



Cellular trophoblast 



Amnion cavity [blast 



Mesoderm lining of tropho- 



Mesodenn of amnion 



Ectoderm of amnion 

 Allantoic diverticulum 

 entoderm vesicle 



Mesoderm covering of 

 entoderm vesicle 



eurenteric canal 



,vity of entodermal vesicle 



FIG. 32. SCHEMA OF SAGITTAL SECTION OF ZYGOTE ALONG 

 LINE A IN FIG. 31. 



Chorion < 



Plasmodial trophoblast 

 Cellular trophoblast 



Neural groove 



perineal section of the posterior part of the primitive streak that, at a later period 

 of embryonic life, the anal and urogenital orifices of the body are formed. 



The Formation of the 

 Notochord and the Secondary 

 Mesoderm. The notochord 

 and the secondary mesoderm are 

 formed from the primitive 

 streak; the notochord from its an- 

 terior extremity, and thesecond- 

 [Body stalk mesoderm ary mesoderm from its lateral 

 ^Extra-embryonic coeiom margins and posterior end. 

 Entoderm As soon as the primitive 



streak is established its anterior 

 end becomes a node or centre 

 of growth by means of which 

 the length and, to a certain 

 extent, the breadth of the body 

 are increased. The portion of 

 the body formed by the activity 

 of the anterior end of the streak 

 is the dorsal portion, from the 

 back part of the roof of the nose, 

 anteriorly, to the posterior end 

 of the trunk. The perineum 

 and the ventral wall of the 

 body, from the perineum to the 

 umbilicus, are formed from the 

 posterior part of the primitive 

 streak. Nevertheless, the 

 primitive streak undergoes little 

 or no increase in length; in- 

 deed, as growth continues, it 

 becomes relatively shorter as 

 contrasted with the total length 

 of the embryonic region, for the 

 new material, formed by its 

 borders and its anterior ex- 

 tremity, is transformed into 

 the tissues of embryo as rapidly 

 as it is created. 



The Notochord. The 

 notochord or primitive skeletal 

 axis is formed by the prolifera- 

 tion of cells from 'the anterior 

 end of the primitive streak. 

 On its first appearance it is a 

 narrow process of cells, the head 

 process, which projects forwards 

 from the anterior boundary of 

 the neurenteric canal, between 

 the ectoderm and the entoderm. 

 Shortly after its appearance the 

 head process wedges its way 

 between the entoderm cells, and 

 from that period onwards, as 

 the posterior parts are formed, 



by continued proliferation from the front end of the primitive streak, they are at 

 once intercalated in the dorsal wall of the entodermal sac, where they remain, 

 forming a part of the dorsal wall of the entodermal cavity (Fig. 33), for a 



V Mesoderm lining of trophoblast 



Amnion cavity 

 Extra-embryonic coeiom 



Mesoderm of amnion 



Ectoderm of amnion 



Mesoderm covering 

 entoderm 



Entoderm 



Cavity of entodermal 

 vesicle 



Notochord 



FIG. 33. SCHEMA OF TRANSVERSE SECTION OP ZYGOTE ALONG 

 LINE B IN FIG. 31. 



Primitive streak 



Primitive groove 



Plasmodial trophoblast 



Cellular trophoblast 



Mesoderm lining of 

 trophoblast 



-Chorion 



Extra-embryonic coeiom 



-. Mesoderm of amnion 



Ectoderm of amnion 



Amnion cavity 



Mesoderm covering 

 "entoderm 



' Entoderm 



^Cavity of entodermal vesicle 



FIG. 34. SCHEMA OP TRANSVERSE SECTION OF ZYGOTE 

 LINE C IN FIG. 31. 



ALONG 



