CHRONOLOGY OF HUMAN DEVELOPMENT. 981 



vesicle, from which the embryo (2.2 mm.), with a closed medullary canal, had 

 begun to be constricted off. 



There now follows the stage in which the formation of the allantois appears. 

 It is at present a much-disputed point whether in man a free allantoic vesicle, 

 growing from the navel, exists or not. The youngest embryo that bears upon 

 this point has been examined by v. Preuschen and the author. In the fresh state, 

 this measured 3.78 mm. in length; it was divided into sections and thoroughly 

 studied. The brain-vesicles were indicated; the organs of special sense were 

 wanting; the ganglia in the cephalic region were visible. The visceral arches were 

 visible as thickenings in cross-section, but not yet isolated; the visceral clefts, 

 the mouth, and the anus were absent. The sella turcica was in process of forma- 

 tion. Heart, lungs and liver were in their earliest form. The umbilical vesicle 

 (torn) was apparently still provided with a wide opening. The allantois was 

 distinct as a free vessel outside of the abdomen; its lamella from the mesoderm 

 was yet without vessels. The extremities were entirely absent. The chorda 

 dorsalis was indicated, and on either side the primitive vertebral masses. 

 A free projecting allantoic vesicle has also been described in embryos by W. 

 Krause and Bruch, but these were older. 



An ovum of from the fifteenth to the eighteenth day has been described by 

 Coste; it was 13.2 mm. long; the villi were small, and slightly branched ; the embryo 

 was 4.4 mm. long, of curved form, with a moderately thickened cephalic portion. 

 Amnion, umbilical vesicle (with a large omphalomesenteric duct), and allantois 

 were fully developed, the last already adherent to the serous covering. The 

 S-shaped heart, lying in the cardiac cavity, exhibits a cavity and the bulb of the 

 aorta, but no ventricles or auricles. The visceral arches and clefts are indicated, 

 but the latter are not yet broken through. Upon the umbilical vesicle the first 

 circulation of the two omphalomesenteric arteries is developed; the fol ding- 

 off is only moderately advanced; the duct is still widely open; two primitive 

 aortas pass in front of the primitive vertebrae. The allantois, adherent to the 

 fetal membranes, possesses its vessels. The two omphalomesenteric veins, 

 united with the two umbilical veins, empty into the lowermost, venous portion 

 of the heart. The mouth is in process of formation. The extremities and organs 

 of special sense are wanting; the Wolffian bodies are probably present. Similar 

 descriptions have recently been made by His, although the length of the embryo 

 was somewhat less. 



There now follows a stage wherein all of the visceral arches are indicated, 

 and the clefts are broken through. The midbrain forms the highest point of the 

 brain; the two auricles appear in the heart. The communication with the um- 

 bilical vesicle is still pretty free. The embryo is from 2.6 mm. to 3.3 or 4 mm. long. 

 The head undergoes a deflection to the side. At a still later period there appear 

 on the brain the parietal and the nuchal curvature; the hemispheres appear more 

 distinct; the entrance to the umbilical vesicle becomes constricted, the rudiment- 

 ary liver is discernible; and the extremities are still absent. In addition to the 

 embryo of His one of the twentieth day described by Johannes Miiller belongs 

 here. The ovum was between 15.2 by 17.6 mm. in size; the embryo from 5 to 

 6 mm. long; the umbilical cord 1.3 mm. thick. The umbilical vesicle was in 

 free communication with the bowel. The amnion surrounded the embryo and 

 formed a sheath for the umbilical cord. The visceral arches and clefts were 

 present; and behind them the projecting heart-tube; the extremities were want- 

 ing. 



Third week (R. Wagner): The ovum measured 13 mm., the embryo from 4 to 

 4.5 mm., the umbilical vesicle 2.2 mm. ; the bowel was almost entirely closed. Three 

 visceral clefts, the Wolffian bodies, the first rudiments of the extremities, three brain- 

 vesicles, and the auditory vesicles were present. A similar embryo described by 

 Hensen should be included here. Twenty-first day (Coste): The nasal pits, the 

 buccal orifice, the eyes, the auditory vesicles, four visceral arches and the mouth 

 (toward which the frontal and the superior maxillary process grow) were especially 

 marked; the heart with two ventricles and two auncles and the vessels of the 

 umbilical vesicle were present. 



End of the First Month. The embryos of from twenty-five to twenty-eight 

 days are characterized by the distinct pedunculation of the umbilical vesicle 

 ancl by the definite appearance of the extremities. The length of the ovum is 

 17.6 mm.; of the embryo from 8 to n mm., of umbilical cord 4.5 mm. with its 

 vessels. 



Second Month. Embryos of from twenty-eight to thirty-five days begin to 



