38 SEGMENTATION AND FORMATION OP THE LAYERS. 



endoderm, but the vacuoles of the endoderm are the largest. 

 There is generally a special layer of vacuoles beneath the ecto- 

 dermal nuclei, between which strands of protoplasm pass from 

 the ectodermal to the endodermal reticulum. 



The blastopore is a slightly elongated structure (PI. I, 

 figs. 19, 21), and is itself traversed by a loose protoplasmic 

 reticulum (PI. V, fig. 24 b). The endodermal layer lining 

 the gut sends out a few processes into the gut which anas- 

 tomose with the blastopore reticulum. The gut of young 

 gastrulse contains a largely developed reticulum (PI. V, fig. 

 23), the remains of the previous stage. In older gastrulse 

 there may sometimes be seen apparently isolated masses of 

 protoplasm (PL V, fig. 24 a), which, however, are probably 

 connected with the endodermal lining and eventually drawn 

 into the latter. 



Just in front of the blastopore there is a large number of 

 nuclei in the middle ventral line (PI. V, fig. 24 a). 



Behind the blastopore there is a special area of ectoderm in 

 the middle line which I have called the polar area, and which 

 possesses the following characteristics : close behind (PI. 

 V, fig. 24 c) the surface is flat and, if anything, marked 

 by a slight groove, the nuclei are more columnar than else- 

 where, and there is a larger quantity of protoplasm outside the 

 nuclei than in most other parts of the ectoderm. Further 

 back (PI. V, fig. 24 d) there is in the middle line a fairly 

 large area of protoplasm containing one or more large round 

 nuclei. 



The polar area extends from the blastopore backwards for 

 a distance in this embryo (figured on PI. I, fig. 21) of about 

 •07 mm. The nuclei in this area will give rise to the nuclei of 

 the primitive streak. 



The protoplasm of the polar area is vacuolated in the ordi- 

 nary way. Fig. 21, PL IV, represents a drawing under 

 a higher magnifying power of the hinder part of the polar 

 area of this stage. 



Figs. 22 a — c, represent a series similar to the above through 

 the polar area of a rather older embryo. The front part of 



