21t> WILLIAM BATESON. 



A section through the blastoporic tract of the same larva is 

 given in fig. 24. The segmentation cavity is here still large, 

 and the walls of the archenteron are found fused with the 

 epiblast at a point at one side of the larva which is otherwise a 

 radially symmetrical, double-walled cylinder. At this point 

 the archenteric cavity also slopes to an angle which persists 

 after the complete separation of the walls from the epiblast. 

 By the persistence of this appearance the point at which the 

 closure of the blastopore is completed can be identified as 

 being nearer to the dorsal surface of the animal, being at the 

 junction of the dorsal and posterior surfaces. The archenteric 

 cavity contains a quantity of coagulum at this and subsequent 

 stages. 



The transverse constriction which now arises (fig. 11, g) lies 

 at the junction of the columnar portion of the hypoblast with 

 the more amceboid part, mentioned above as being found in the 

 anterior end. It is, at first, merely a groove in the epiblast, 

 which coincides with a similar depression in the hypoblast. 

 As will presently be described, the walls of that part of the 

 archenteron which lies in front of this groove become con- 

 stricted off to form the unpaired anterior body cavity ; and the 

 commencement of the consequent histological differentiation 

 may be already perceived in the more amoeboid character of 

 the hypoblast cells in this region (fig. 25, H'). 



The internal relations of the parts between Stages C and D 

 are shown in fig. 25, which is constructed from the same series 

 of transverse sections from which figs. 23 and 24 were taken. 



In Stage D the hypoblast separates completely from the epi- 

 blast, and the appearances to be described in Stage E begin to 

 be present. 



Stage E — Formation of the Mesoblast. — Leaving the 

 larva in the form of the closed, two-walled cylinder described 

 above, whose cavity has already begun to become constricted 

 into two parts, the anterior being lined by somewhat amoe- 

 bold, hypoblastic cells, and the posterior by regular, columnar 

 hypoblast, a description of the changes to be seen in Stage E 

 will be given in detail. 



