CTENOPHORA 



143 



through Metschnikoff (No. 16), they are also destined to 

 supply the elements of the mesoderm. For in the present 

 stage (Fig. 67 A) there is effected a new abstriction from 

 the macromei-es of small elements (me), which we may 

 designate as mesoderm cells. At first these form a cell-plate, 

 which lies at the lower, free surface of the macromeres, 

 when the latter are not yet covered by ectoderm. But in 

 the stages which now follow (Fig. 67 B and C) certain im- 

 portant changes are accomplished by means of which this 

 fundament soon reaches the inside of the embryo. In this 

 connection we must first glance at the upper pole of the 

 embryo (Fig. 67 A). Here the embryo still exhibits a small 

 opening, which in earlier stages (Fig. 66 B and G) was 

 larger and is to be referred to the inner circumfei-ence of 



Fig. 67. — Three embryos of CalUanira hialata in transverse section, diagram- 

 matic (after Metschnikopp, from Lang's Le?irbMc7i). ec, ectoderm; eii, entoderm; 

 me, mesoderm J d, intestinal cavity ; st, oesophagus (fundament of stomach). 



the ring of micromeres (Fig. 65 I, K, L). This opening in 

 the stage of Fig. 67 A is still in connection with a cavity 

 existing between the macromeres, which we recognize as 

 the remains of the cleavage cavity. Both the cleavage 

 cavity and the iipper opening in the ectoderm, which has 

 been erroneously assumed by many authors to be the blasto- 

 pore, now disappear by the neighbouring cells closing tightly 

 together. At the same time an invagination of the lower 

 surface of the macromeres and of the adjacent mesodermal 

 plate (me) is eifected ; by means of this a cavity (gastrula 

 cavity) is formed opening downwards, the lower portion of 

 which is lined with entoderm cells, the upper portion with 

 mesodermal elements (Fig. Q7 B). In the further course of 



