SEGMENTATION. 23 



cells. This is the expression of the looser part of the reticulum, 

 which is continuous internally with the reticulum of the 

 adjoining cells, and externally with the clearer masses consti- 

 tuting the rest of the ovum, and called by me the endoderm 

 masses (PI. IV, fig. 19). All the above elements are 

 arranged round the central cavity, which was present even in 

 the unsegmented ovum. Fig. 14, PI. IV, is a diagram- 

 matic representation of a transverse section through the ecto- 

 derm cells at this stage ; it shows the continuity of the looser 

 circumferential parts of the reticulum of the two cells (the 

 endoderm masses are not represented in this figure). 



The next divisions take place parallel to the long axis of the 

 ovum, and result in the formation of sixteen ectoderm cells 

 arranged in four rows, each row containing four cells. A 

 diagrammatic transverse section of such an ovum is shown ia 

 fig. 15, PI. IV, in which the endoderm masses are repre- 

 sented. This section also shows the segmentation cavity 

 around which the various elements are arranged. 



The further changes which may be considered as belonging 

 to the segmentation stages consist in the continued and regular 

 subdivision of the ectoderm cells, and in the continued breaking 

 up of the endoderm masses into smaller bodies. Fig. 8, 

 PI. I, represents a fully segmented ovum. It consists of a 

 small patch of ectoderm cells, and a number of irregular 

 branched endoderm masses. Both the ectoderm cells and 

 endoderm masses are placed immediately beneath the egg- 

 membrane round the segmentation cavity. A diagrammatic 

 representation of a transverse section of such an ovum is shown 

 in PI. Ill, fig. 10, and PI. IV, fig. 17 is a drawing of an 

 actual section through such an ovum in situ in the uterus. 



The reticulum which connects the endoderm masses is shown 

 — highly magnified — in fig. T, PI. III. It lies immediately 

 beneath the egg-shell and consists of pale, hyaline strands, 

 which at the nodes spread out into flat expansions. The strands 

 contain a small number of strongly refractile globular bodies. 

 This drawing was made from an uninjured ovum preserved 

 in sublimate and acetic acid. The reticulum connecting the 



