80 



GENERAL HISTOKY OF DEVELOPMENT 



GENERAL, HISTORY OF THE DEVELOPMENT OF 

 THE HUMAN EMBRYO. 



Estimation of ag-e. In estimating the age of embryos prematurely expelled 

 from the uterus, we must, in the absence of data as to the early stages, have recourse 

 to a conventional rule. The rule generally adopted is that formulated by His. 

 It reckons the duration of pregnancy from the first day of the first omitted 

 period, the cessation of the menses being the earliest positive sign of 

 impregnation. ' 



First month. The ova of Leopold, of Peters, of Beneke, and one of 

 those described by Graf v. Spee, must, according to His' rule, be reckoned 

 to belong to the first few days of pregnancy. Leopold's ovum is probably the 

 youngest yet discovered, but the relations were somewhat disturbed by intense 

 congestion of the decidua, and no embryonic rudiment was identified. 



mes am 



em.pl. 



y.s. 



FIG. 109. SECTION OF EMBRYONIC RUDIMENT IN PETERS' OVUM (FIRST WEEK). (After Peters.) 



ect, ectoderm of chorion ; mes, mesoderm ; am, amnion ; em.pl., embryonic plate ; y.s., yolk-sac ; 

 ent, entoderm ; ex.cce, portion of extra-embryonic coelom limited by a strand of the magma reticulare. 



We therefore begin with Peters' ovum. In this the cavity of the chorionic 

 vesicle measures 1-6, '8, and '9 mm. in its three diameters. It is entirely surrounded 

 by very irregular trophoblast- strands, and the mesoderm is beginning to extend 

 into these strands to form the primitive villi. The embryonic rudiment (fig. 109) 

 is still a single layer of ectoderm, to which is attached a small yolk-sac, while it it 

 covered by a closed amnionic sac. The yolk-sac and amnion are contained withii 

 and attached to the completely closed chorion, by mesoderm which is alreadj 

 separated into visceral and parietal layers. The extra-embryonic coelom thi 

 formed is relatively very large, and is intersected by strands of fibrillae knowi 

 as the magma reticulare. 



The next stage may be illustrated by Spec's embryo v H (fig. 110) 

 belonging to the beginning of the second week. The chorionic vesicle measures 



1 Mall gives a useful formula for estimating the age of human embryos up to 100 mm. in length : 

 v/lOOxTength in mm. = age in days. In foetuses measuring from 100 to 220 mm. the vertex-breech 

 millimetre length is approximately equal to the age in days. 





