THE CLASSIFICATION OF THE EARLY STAGES. 119 



at the same stage. Moreover, the variability of embryos is very great, for in 

 specimens otherwise alike we find this or that organ advanced or retarded in its 

 development as compared with the embryo as a whole. Nevertheless it is possible 

 with the information at command to determine with tolerable certainty the age of 

 an embryo within two days plus or minus, up to the end of the ninth week. For 

 the course of development during the third month we possess as yet no satisfactory 

 data, but embryos of full three months are quite frequently obtained, and are very 

 characteristic in size and configuration (see page 156). 



F. P. Mall's formula for calculating the age of human embryos is 



A/ioo X length in mm: 

 The length is measured from the vertex to the breech. 



The Classification of the Early Stages. 



Any attempt to divide embryos into stages must necessarily establish artificial 

 groups, for in nature there is no demarcation. Division into stages is for con- 

 venience, and ought, therefore, to be made by natural and obvious characteristics. 

 It seems to me that eleven stages may be conveniently discriminated, as follows: 



First Stage. Segmentation of the Ovum: The general process is described on 

 pages 42 to 45. There are no observations upon this stage in man, or any primate, 

 except one monkey's ovum in the four-cell s age described by Selenka. 

 * Second Stage. Blastodermic Vesicle: The general development of the blasto- 

 dermic vesicle in mammals is described on page 45. Its development in man is 

 unknown. During this stage the embryonic shield is differentiated. An ovum 

 of a monkey in this stage is described on page 127, and one of the very few 

 known human ova is described on page 128. 



Third Stage. Primitive Streak : Two human ova with a primitive streak be- 

 fore the formation of the medullary plate have been observed. 1 In one of these, 

 Frassi's embryo, the diameter of the entire ovum was 13 x 5 mm. The diameter 

 of. the yolk-sac 1.9 x 0.9 mm. The embryonic shield was 1.17 mm. long by 0.6 

 mm. wide. The primitive groove is shallow and occupies about half the length 

 of the shield. The anterior end of the groove marks the position of the future 

 neurenteric canal; its posterior end, the position of the anal plate. 



Fourth Stage. The Medullary Plate: In this stage there are several embryos 

 known. In all of them the amnion and chorion are already differentiated. There is 

 a large extra-embryonic coslom. The chorionic vesicle is rounded and somewhat 

 flattened. In its greatest diameter it measures from 8 to 10 mm. It is beset 

 with short branching villi which are present over the entire surface. The general 

 relations are indicated in the accompanying diagram (Fig. 69). The chorion has a 

 distinct epidermal and mesodermal layer. To its inner surface is attached the 

 body-stalk wihch unites the embryo and chorion. From it springs the amnion 



'The Harvard Embryological Collection has an embryo, Series 825, in fine preservation. It is a little younger 

 than Frassi's. It is hoped to publish an account of it soon. 



