22 SEGMENTATION AND FOEMATION OP THE LAYERS. 



opaque portion, whicli contains the nucleus and is closely 

 applied to the opaque part of the other segments (PI. I, fig. 4). 

 A careful examination of this ovum shows that the furrow has 

 not completely separated the two segments from each other, 

 but that they are connected by strands of protoplasm forming 

 a loose network between them. This network is simply a 

 looser part of the ordinary protoplasmic network described at 

 the beginning of this chapter. There are, however, no such 

 strands between the most superficial parts of the opaque areas; 

 in this region the furrow is for the moment complete. Soon, 

 however, the clearer protoplasm (where the network is looser 

 and continued into the still looser network between the two 

 segments) extends upwards on the inside of the dark patch, so 

 that when four segments are formed by the second vertical 

 furrow each dark patch is surrounded on all sides by a 

 layer of the looser reticulum (PL I, fig. 5), which is here 

 as elsewhere continuous with the reticulum of the adjoining 

 segments. 



Two changes now occur : (1) the pale, clearer, and larger 

 part of the four segments begins to break up into smaller, 

 irregular masses of varying size, which, however, are seen on 

 careful examination to be connected with each other by a wide- 

 meshed reticulum, and (2) a third furrow appears dividing the 

 four dark patches, which I have called the ectoderm cells, into 

 eight patches or cells (PI. I, fig. 7). This furrow may be 

 looked upon as corresponding to the horizontal furrow, which 

 ordinarily follows the second vertical furrow in the segmenta- 

 tion of the ovum. The ovum therefore now consists of eight 

 ectoderm cells, and four large and a number of smaller endo- 

 derm masses, all connected together by a wide-meshed reticulum, 

 and placed immediately beneath the egg-shell around a central 

 cavity — the segmentation cavity. Each ectoderm cell 

 presents in the fresh specimen (PI. I, fig. 7) (1) a central 

 clear area — the expression of the nucleus; (2) around this a 

 dark area — the expression of the dense protoplasmic reticulum 

 around the nucleus ; and (3) a paler circumferential area, which 

 is more marked on the outer than on the inner border of the 



