250 AYERS ON THE DEVELOPMENT 



cells assist in forming the walls of the blood canal and do not become blood corpuscles. 

 There are given off from either side of the heart two thin sheets of mesoderm (pi. 23, fig. 2), 

 one above and one below the middle line. These unite at a short distance from the vessel 

 on either side, but immediately diverge again into what appear, upon cross section, to be a 

 pair of more or less circular vessels extending in a plane parallel to, but below, the heart. In 

 the membrane thus stretched between these vessels and the heart there are usually a number 

 of nuclei having the same size and appearance as those in the wall of the heart. From 

 the outer sides of these lateral tubes, mesodermic elements diverge in the form of a vary- 

 ing number of sheets (in some instances there are as many as four, but more commonly 

 only two), which join the somatic layer of the mesoderm. After the formation of the 

 heart there appears in the thoracic and maxillary regions below the nervous cord a tube of 

 about the size and shape of the dorsal vessel. (PI. 21, fig. 41.) It is a ventral blood sinus 

 similar to the two latero-dorsal tubes just described. In the posterior abdominal region, 

 about the time of the appearance of the Malpighian vessels, the heart is connected with 

 the proctodaeum by a dorsal mesentery similar in structure to the sheets just described. 

 In embryos treated with osmic-acetic acid the heart, like the body cavity, is at this time 

 found to be filled with a finely granular substance, probably coagulated plasma, which 

 appears grayish in reflected light and is faintly stainable. (PI. 22, fig. 10.) During the 

 contraction of the yolk sac most of the nuclei of the serosa cells which are then set free 

 pass into the mesenteron, but, as has been stated in a previous paragraph, some of them 

 pass into the body cavity and after further transformation find their way into the heart. 

 These serosa nuclei become large and vesicular during the decline of the yolk sac, and 

 their nuclear substance breaks up into several irregular masses connected together by a 

 few coarse filaments and numerous granules. In this stage they are freed from the sur- 

 rounding cell, the protoplasm of which during this time has become thin and watery. 

 The nuclear membrane is seen to be a delicate, structureless, scarcely stainable vesicle 

 surrounding the nuclear substance. If the nucleus passes into the mesenteron with the 

 yolk, it does not undergo any marked transformation and may either disintegrate or pro- 

 liferate and form, by collecting protoplasm about itself, from one to several amoeboid 

 endodermic cells. If, on the other hand, the nucleus passes into the body cavity (pi. 22, 

 fig. 1) it becomes more vesicular, its membrane much thinner, while nearly all of the 

 stainable substance is promptly concentrated into the nucleolar masses, and ultimately all 

 of the nuclear substance goes to form from one to three spherical bodies which are sur- 

 rounded by the common membrane. These bodies are blood corpuscles and are free 

 nucleoli immediately on the rupturing of the vesicle which surrounds them (PI 22 fi^s 

 1 and 3.) ' ' h ' 



The invaginations of the ectoderm which form the tracheae do not occur so early in 

 Oecanthus as they do, according to Kowalevski(26), in Hydrophilus, neither do they occur 

 on the ventral surface of the segments, as Kowalevski has represented in his pi. 8, fig. 10. 

 On the contrary, one does not find the tracheal pockets until after the embryo has revolved 

 in the egg and the dorsal wall is partly closed. They then appear as invaginations of the 

 pleural region. 



There are to be seen in sections of the head segment, at the time when the invaginations to 

 form the salivary glands are well advanced, small infoldings of the ectoderm similar in posi- 



