30 



HUMAN EMBKYOLOGY. 



After the separation of the scleratogenous segments of the mesodermal somites, 

 the remainders of the somites, each of which consists of a flat plate with incurved 

 dorsal and ventral margins, constitute the muscle plates from which the striped 

 muscle fibres are derived. 



In the opinion of some observers the outermost portion of each of the above -described plates 

 is developed into subcutaneous connective tissue cells ; consequently it is spoken of as the cutis 

 lamella. According to this view the muscle cells are formed from the innermost cells and the 

 incurved margins of the plates. 



The Intermediate Cell Tracts. The intermediate cell tracts are the rudiments 

 of the internal organs of the genital system and the temporary and permanent 

 urinary system, with the exception of the urinary bladder and the urethra. 



The Lateral Plates. From the cells of the lateral plates are formed the lining 

 endothelial cells of the great serous cavities of the body the pleurae, the peri- 

 cardium, and the peritoneum ; the majority of the connective tissues, with the 

 exception of those of the vertebral column and the head, the greater part or 

 all the mesoderm of the limbs, and, probably, the unstriped muscle fibres of the 

 walls of the alimentary canal and the blood-vessels. 



Position of otic 

 vesicle 



Neural crest 



Trophoblast of chorion 



iraxial mesoderm 



Embryonic 

 arc 



Intermediate cell tract 

 Splanchnic and 

 somatic layers of 

 'lateral plate meso. 



_Etnbryonic 

 coelom 



Amnion 



Neural crest 

 Caudal neuropore 



Notochord 



Mesoderm of 



yolk-sac / 

 Entoderm of yolk-sac 



S^xtra-em- 

 bryonic coelom 



Alimentary canal 

 Yolk-sac 



FIG. 43. 



A. Diagram of a transverse section of a zygote, in which the neural tube has formed but has not separated 



from the surface ectoderm. 

 B. Diagram of embryonic area of same zygote. Compare with surface view of embryo in Fig. 38. 



The Cephalic Mesoderm. It has already been noted that the mesoderm of the 

 head becomes segmented only in the region of the caudal part of the hind-brain, 

 where four cephalic mesodermal somites are formed on each side. From the 

 scleratogenous portions of these somites are developed the occipital part of the skull 

 and the corresponding portions of the membranes of the brain, and from their 

 muscle plates the intrinsic muscles of the tongue. 



The unsegmented part of the cephalic mesoderm gives rise to the remaining 

 muscles and connective tissues of the head region. 



Early Stages of the Development of the Nervous System. No definite trace 

 of the nervous system is present until the primitive streak has formed and the 

 embryonic area has passed from a circular to an elongated form. Then an area of 

 thickened ectoderm, the neural plate, appears in the anterior part of the embryonic 

 area. It commences a short distance posterior to the anterior end of the area, 

 and its posterior extremity embraces the anterior end of the primitive streak. Its 

 lateral margins fade into the surrounding ectoderm, and, in the earliest stages, 

 cannot be definitely defined ; but, as the elongation of the plate continues coinci- 

 dently with the elongation of the embryonic area, the lateral margins of the plate 

 are elevated as the mesoderm beneath them thickens, and so they become distinct. 



