420 MARGARET ROBINSON. 



it is ready for absorption by the eudoderm they (having done 

 their work) disappear. The appearance of yolk in preserved 

 embryos is^ of course, to a certain extent influenced by the 

 fixative used. 



Mesoderm. — There is an increase in the number of 

 mesoderm cells, but as yet they show no very definite 

 arrangement except in the region of the stomodaeum where 

 they are beginning to form a layer one cell deep round the 

 stomach (fig. 61, mes.). 



Figs. 59 and 60 show mesoderm cells running in oblique 

 lines dorsalwards from the dorsal wall of the stomodifium. 

 These lines doubtless represent the bands of connective tissue 

 and muscle which in later stages run from the stomach and 

 oesophagus to the dorsal body-wall. 



In the region of the second maxilla, beginning at the 

 posterior end of its ganglion, there are on either side 

 stretching outwards from the ganglion towards the shell- 

 thickening, three mesoderm cells which form a rudimentary 

 muscle (fig. 62). The figure represents a transverse section 

 taken slightly behind the ganglion. I have not been able to 

 trace fibres from the ganglion to the muscle, but have little 

 doubt that it is innervated from this ganglion (that of the 

 second maxilla). Therefore it does not foreshadow the great 

 transverse shell muscle of the adult which is innervated from 

 ganglion of the first maxilla (Claus, 1889). 



The Heart. — Of the seven mesoderm cells which were 

 present under the dorsal ectodei'mal thickening in the hist 

 stage, I can find but one here (fig. 61). However I have 

 little doubt that those cells do represent the heart in its 

 earliest stage, for in a stage which is but very little older 

 than this (Stage F) one sees in the same region in which this 

 baud of cells was present in Stage E (i. e. the region near the 

 posterior end of the stomoda3iim) a few mesoderm cells 

 arranged round a definite lumen just under the dorsal shell- 

 thickening (figs. 55 a, 55?;, 55 c). This must be the anterior 

 end of the heart or dorsal aorta. I imagine this to have been 

 formed by a bending inwards of the mesoderm cells so as to 



