A HUMAN EMBRYO OF THE PRESOMITE PERIOD. 421 



tion of that process; that is, development is regarded as arrested 24 hours before 

 the completion of the abortion. 



The Mateer specimen is probably not younger than the Reichert specimen, 

 which in turn can not be younger than 15 days. On the other hand, it is distinctly 

 younger than the Eternod specimen, which is not older than 19 days. If the Ingalls 

 and Grosser specimens, which are also younger than the Eternod specimen, are 

 placed at 18 days, then the Reichert, Giacomini, and Mateer specimens, being still 

 younger, would fall back on the 17th day, which is probably very close to their 

 actual age. 



On examining chart 3 it will be found that, if the age-deductions are correct, 

 pregnancy in 16 of the cases listed began respectively 1, 1, 3, 4, 5, 9, 10, 11, 13, 13, 

 14, 14, 17, 18, 19, and 20 days following the end of the last menstrual period. 1 In 

 no case did it begin during the period or in the 4 days immediately preceding it. 

 It would appear that conception takes place most frequently (25 per cent of the 

 above cases) at about the 13th or 14th day following the cessation of the menstrual 

 period. In nearly one-half of the cases it occurred at the end of the second and 

 during the third week, which has previously been regarded as a comparatively 



sterile period. 



TWIN EMBRYO. 



Before concluding I wish to call particular attention to the two small vesicles 

 which are to be found in the region of the body-stalk, mention of which was made 

 under the serial description of sections (sees. 68 to 79). The opinion was there 

 expressed that these structures constitute respectively the amniotic vesicle and yolk- 

 vesicle of another embryo. Inasmuch as an opportunity is thus afforded to observe 

 twin-formation at an extremely early stage, this specimen has an important bearing 

 upon the problem of that process. 



As we have seen, intervening between the amnion and the chorionic membrane 

 there is an area which is partially filled with parietal mesoblast. Caudally, this 

 area surrounds the body-stalk proper, which can be distinguished by its more com- 

 pact structure. Around the base of the body-stalk the parietal mesoblast walls 

 itself in from the exoccelomic cavity by a membranous arrangement of its super- 

 ficial cells in the form of a mesothelium. The latter flares outward and becomes 

 indistinct as it approaches the chorionic membrane, so that in this region the 

 parietal mesoblast is more irregularly marked off from the exocoelom, constituting 

 an area of scattered mesoblastic tissue in which the exoccelomic development is not 

 yet complete. 



A section through this region is shown in figure 37, plate 6, in which will be 

 recognized the chorionic membrane above and a portion of the yolk-sac of the main 

 embryo and its abdominal stalk below. In the loose tissue lying between these are 

 the two detached vesicles, which together constitute the twin. The smaller, with a 

 lumen 0.03 mm. in its largest diameter, is interpreted as the yolk-sac. It is shown 

 under greater enlargement in figure 36; here it can be seen that its wall is made up 



1 In the Linzenmeier specimen, owing to the excessive and irregular character of the menstruation, the data could 

 not be safely used. 



