CHEONOLOGIOAL TABLE 



OP 



THE DEVELOPMENT OF THE FCETUS. 



(FROM BEAUNIS AND BOUCHARD.) 



First Week. During this period the ovum is in the Fallopian tube. Having bsen fertilized in 

 the upper part of the tube, it slowly passes down, undergoing segmentation, and reaches 

 the uterus probably about the end of the first week. During this time it does not undergo 

 much increase in size. 



Second Week. The ovum rapidly increases in size and becomes imbedded in the decidua, so 

 that it is completely enclosed in the decidua reflexa by the end of this period. An ovum 

 believed to be of the thirteenth day after conception is described by Reichert. There was 

 no appearance of any embryonic structure. The equatorial margins of the ovum were beset 

 with villi. but the surface in contact with the uterine wall and the one opposite to it were 

 bare. In another ovum, described by His, believed to be of about the fourteenth day, 

 there was a distinct indication of an embryo. There was a medullary groove bounded by 

 folds. In front of this a slightly prominent ridge, the rudimentary heart. The amnion 

 was formed and the embryo was attached by a stalk, the allantois, to the inner surface of 

 the chorion. It may be said, therefore, that these parts, the amnion and the allantois. and 

 the first rudiments of the embryo, the medullary groove, and the heart, are formed at the 

 end of the second week. 



Third Week. By the end of the third week the flexures of the embryo have taken place, so 

 that it is strongly curved. The protovertebral disks, which begin to be formed early in the 

 third week, present their full complement. In the nervous system the primary divisions 

 of the brain are visible, and the primitive ocular and auditory vesicles are already formed. 

 The primary circulation is established. The alimentary canal presents a straight tube com- 

 municating with the yolk-sac. The branchial arches are formed. The limbs have appeared 

 its short buds. The Wolffian bodies are visible. 



Fourth Week. The umbilical vesicle has attained its full development. The caudal extremity 

 projects. The upper and the lower limbs and the cloacal aperture appear. The heart sep- 

 arates into a right and left heart. The special ganglia and anterior roots of the spinal 

 nerves, the olfactory fossae, the lungs and the pancreas can be made out. 



Fifth Week. The allantois is vascular in its whole extent. The first traces of the hands and 

 feet can be seen. The primitive aorta divides into aorta and pulmonary artery. The duct 

 of Miiller and genital gland are visible. The ossification of the clavicle and the lower jaw 

 commences. The cartilage of Meckel occupies the first post-oral arch. 



Sixth Week. The activity of the umbilical vesicle ceases. The pharyngeal clefts disappear. 

 The vertebral column, primitive cranium, and ribs assume the cartilaginous condition. The 

 posterior roots of the nerves, the membranes of the nervous centres, the bladder, kidney, 

 tongue, larynx, thyroid body, the germs of teeth, and the genital tubercle and folds 

 are apparent. 



Seventh Week. The muscles begin to be perceptible. The points of ossification of the ribs, 

 scapula, shaft of humerus, femur, tibia, palate, and upper jaw appear. 



Eighth Week. The distinction of arm and forearm, and of tnigh and leg, is apparent, as well as 

 the interdigital clefts. The capsule of the lens and pupillary membrane, the interventricu- 

 lar and commencement of the interauricular septum, the salivary glands, the spleen, and 

 suprarenal capsules are distinguishable. The larynx begins to become cartilaginous. All 

 the vertebral bodies are cartilaginous. The points of ossification for the ulna, radius, fibula, 

 and ilium make their appearance. The two halves of the hard palate unite. The 

 sympathetic nerves are now for the first time to be discerned. 



1 [Eternod (Anat- Anzeiger, Band xv., 1898) described an ovum which he reconstructed. It 

 had a precise history, from which he concluded that it must have belonged to the end of the second 

 or the beginning of the third week. Including the villi it measured 10 X 8.2 X 6 mm. It was 

 flattened on its embryonal side, and the embryo measured 1.3 mm. The amnion was completely 

 formed and the allantois existed as a long canal. The vitelline circulation was established and the 

 villi of the chorion were beginning to be vascularized. The blastopore still opened" into the airmiotic 

 cavity, with the primitive groove behind it and the rudimentary groove in front. The notochord 

 was closing in and all three layers of the blastoderm were distinct, except around the blastopore, where 

 they formed an undivided mass. EDS.] 



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