36 ALEXANDER MEEK 



dorsal lip of the blastopore. The changes which take place are 

 illustrated by photographs of sections. 



Passing over the stage of completed delamination, figs. 1 and 2 

 (PI. 1) are taken from slides 4 and 12 of a series of horizontal 

 sections of an embryo with a blastopore and invaginated dorsal 

 lip (Text-fig. 2). The primitive enteron is, as will be observed, 

 abstracting food material from the yolk-cells, and the yolk- 

 cells are being changed and broken down in the process. As 

 a result, the anterior wall is thinner than in the previous stage. 

 The blastopore is ushered in as an area of proliferation uniting 

 ectoderm and endoderm. It is later that the groove appears 

 dorsally, and still later that it extends ventrally to form 

 a circular lip. During the earlier period the activities of the 

 blastopore are mainly confined to the dorsal lip, and in that 

 region the blastopore is concerned not only in producing the 

 secondary enteron but in yielding mesoderm wdiich presses 

 outwards on each side separating ectoderm from endoderm. 



Fig. 3 (PI. 1) is a typical sagittal section of a somewhat 

 later stage (Text-fig. 3), but it is scarcely necessary to point 

 out that in this series the anterior end has unfortunately 

 become flattened. The anterior wall may now be said to 

 consist of ectoderm. In association with the invagination of 

 the dorsal lip of the blastopore a margin of endoderm projects 

 anteriorly, providing for the forward extension of the enteron. 

 The margin, however, also advances all round the primitive 

 enteron, thus producing a cup-shaped cavity of the endoderm. 

 The condition defines more clearly the layers and may be 

 described as a reversion to a segmentation cavity under the 

 influence of the blastopore. But, as has already been stated, 

 this condition of the primitive enteron is a temporary one. 



Figs. 4 to 8 (PI. 1) are from successive slides of a series 

 of sagittal sections bearing the numbers 4 to 7 and 9 (Text- 

 fig. 4). These show that the invagination which produces the 

 secondary enteron is confined to the dorsal aspect of the 

 embryo, and that the margins of the cup of endoderm are 

 rapidly approaching and fusing at the anterior pole of the egg. 

 But, more importantly, they indicate that the invagination 

 of the secondary endoderm is accompanied by a breaking down 



