VIII ARTHEOPODA 257 



of celloidin no violent currents are set up, and the most delicate 

 structures are permeated and held in place. Whilst, therefore, 

 not denying Hirscliler's statement as to the communication of the 

 coelomic cavities with the epineural sinus, we are disposed to suspend 

 judgment on the matter till it has been reinvestigated by the aid of 

 more refined methods. It is to be noted that the excretory tubes of 

 Peripatus were confidently stated to open into the blood-spaces of 

 the general body-cavity, till Sedgwick, by the aid of very carefully 

 prepared sections, proved the contrary. 



The coelomic sacs eventually reach the mid-dorsal line, and the 

 cardioblasts of the two sides, which have united previously with their 

 successors and predecessors into continucfus strips of tissue, join with 

 one another to form the tubular heart. Throughout the greater part 

 of its length this union takes place at first dorsally, so that the heart 

 tube is for some time open to the yolk on its ventral side, but in the 

 hindermost region of the embryo the two lines of cardioblasts unite 

 at first ventraUy and then dorsally, so that in this region the dorsal 

 wall of the heart is first formed by the ectoderm. Those portions of 

 the outer wall of the coelomic sacs which lie immediately beneath 

 the cardioblasts, unite with the corresponding pieces of the coelomic 

 sacs of the opposite sides to form the pericardial septum. Cells 

 budded partly from these strips and partly from the yolk form the 

 pericardial strings of cells which lie beneath the pericardium. 



The anterior aorta, the only definite artery which insects possess, 

 is formed by the union in the mid-dorsal line of the two coelomic sacs 

 which belong to the intercalary segment. The aorta becomes filled 

 with blood cells which are derived from the loose endoderm cells at 

 the epineural sinus, they wander up at the sides of the yolk and get 

 between the two apposed rows of cardioblasts. Cardioblasts are not 

 found in the last two abdominal segments. 



The first trace of the genital cells made its appearance coinci- 

 dently with the formation of the blastoderm, as a posterior thicken- 

 ing in that structure. When the "gastral groove" is formed this 

 thickening separates from the overlying blastoderm and moves 

 forwards, so that it is found later at the level of the tenth abdominal 

 segment. Then it divides into right and left halves, and the cells of 

 each half become to a certain extent loose from one another. This 

 looseness is an indication that the cells composing each heap are 

 actively migrating forwards, and a little later they are found pene- 

 trating the mesoderm in the region of the ninth segment ; still later 

 they are found as far forward as the seventh segment. When the 

 coelomic cavities have appeared the genital cells penetrate into the 

 "genital ridge," and in the region of the seventh segment they 

 increase in number and form a cylindrical mass of cells, the rudiment 

 of the genital organ. 



The first beginnings of the central nervous system appear as two 

 longitudinal thickenings of ectoderm, in the stage when the embryonic 

 area is first definitely divided into segments. These thickenings 



VOL. I ^ 



