GROWTH OF THE EMBRYO. 



97 



Fig. 59. 



be the rudiment of the heart. No distinct trace of the omphalo-mesenteric 

 vessels could be observed. 1 



In an embryo of fifteen to eighteen days, described by Coste, the villi of the 

 chorion were well formed, the umbilical vesicle communicated largely with the 

 intestine, and the allantois was present; united to 

 the inner surface of the chorion, and communicat- 

 ing by a large pedicle with the intestine. Both 

 the allantois and umbilical vesicle were vascular. 

 The amnion was not yet closed. 



In ova of the third and fourth week the amnion 

 has been found closed, the rudiments of the eye, 

 ear, maxillary projections, pharyngeal arches, cere- 

 bral vesicles, anterior and posterior extremities, 

 liver and umbilical cord are observed (Fig. 59). 



The further development of the embryo will 

 perhaps be better understood if we follow as briefly 

 as possible the principal facts relating to the chief 

 the cranium, the pharyngeal-cavity, mouth, &c., the 

 nervous centres, the organs of the senses, the circu- 

 latory system, the alimentary canal and its append- 

 ages, the organs of respiration, and the genito- 

 parts of which the body consists, viz., the spine, 

 urinary organs. 2 The reader is also referred to the 

 table of the development of the fcetus on page 112. 



Human embryo in the fourth week. 

 1. Amnion removed in part of the 

 dorsal region. 2. Umbilical vesicle. 

 3. Omphalo-mesenteric duct. 4. In- 

 ferior maxillary tubercle of first pha- 

 ryngeal arch. 5. Superior maxillary 



tubercle from the same arch. 6. 

 Second pharyngeal arch. 1. Third. 

 8. Fourth. 9. Eye. 10. Primitive 

 auditory vesicle. 11. Anterior ex 

 tremity. 12. Posterior extremity. 

 13. Umbilical cord. 14. Heart. ]5- 

 Liver. 



Development of the Spine. The first trace of the 

 future spinal column is found at a very early period 

 of foetal life, constituting the chorda dorsalis or 

 notochord (Fig. 53). This is a cylindrical tube, 

 composed of a transparent sheath, containing em- 

 bryonic cells, and extending from the cephalic to 

 the caudal extremity of the fcetus below the spinal canal. The proto-vertebrse or 

 primitive vertebrae appear early, as dark spots, which soon enlarge and form 

 quadrangular laminae, one on either side of the chorda dorsalis, commencing in 

 the cervical region. These spread out and bend towards each other, so as to 

 come into contact around the spinal canal and inclose it, forming the rudiment 

 of the future bodies and arches of the vertebrae, as well as of the vertebral and 

 other muscles. This primitive vertebral column is, however, entirely mem- 

 branous until about the sixth or seventh week, when cartilage begins to be 

 deposited in it. The proto- vertebrae do not coincide with the permanent verte- 

 brae. On the contrary, each primitive vertebra separates into two parts, the 

 upper part belonging to the permanent vertebra, which lies above the point of 

 separation, and the lower one to that below. The chorda dorsalis becomes 

 gradually atrophied, except at the part corresponding to the intervals between 

 the permanent vertebrae, where it forms the intervertebral disks. (The particular 

 facts relating to the ossification of the spinal column will be found under the 

 description of the Vertebrae.) 



Development of the Cranium in general, and of the Face. The foetal cranium 

 is developed from the primitive vertebral disks surrounding the upper extremity 



1 A third early embryo is figured and described in this paper, but the author is more uncertain 

 as to its date. 



2 The scope of this work only permits the briefest possible reference to these subjects. Those 

 who wish to study the subject of embryology in more detail are referred to Kolliker's Entwicke- 

 lungsgtschichte, to the chapters on the development of the various organs in the 7th edition of 

 Quain's Anatomy, or to Beaunis et Bouchard, Nouvc.aux Elements d'Anatomie descriptive et 

 a'Eiubryologie ; to the latter of which works especially the editor must express his obligations. 



7 



