MEMOIRS OF TIIK NATIONAL ACADEMY OP SCIENCKS. 441 



The paired structure of the eetodermal plate is well shown in the antennnlar ganglion on a 

 level with the tran.sver.se commissures, or even in front of this, where paired masses, \\illi small, 

 deeply dyed nuclei, are separated by a median sheet of much larger and clearer cells. This ma\ 

 possibly correspond to the mittelstrang, referred to again. 



Shortly after this (Fig'. 130) the ganglia are blocked off by a series of superficial constrictions. 

 At least seventeen such gauglionic segments can be counted, beginning with the optic; and supra- 

 oesophageal ganglia and passing to the last abdominal segments. The ganglionic blocks are 

 formed rapidly from the front backward. The ganglia of the first antenna- are now the most con- 

 spicuous part of the nervous system, unless we accept the large optic ganglia. There is a broad, 

 transverse, fibrous commissure in the antennular segment, which is still more prominent at a little 

 later period (PI. XLVI), when eye pigment is forming. From this commissure longitudinal rods 

 extend forwards and unite the brain with the optic ganglia, while similar rods grow backward and 

 form the fibrous axis of the circum-oesophageal commissures. 



The plane of section in Fig. 148 passes just in front of the oesophagus and through the roots 

 of the first pair of antennae (A 1), which should appear in the drawing as cunt in nous with the 

 integument. The ganglionic cells, which are directed toward the appendages, represent tiie anten- 

 nular nerves, and are more apparent in the following section. The antennular ganglion is both 

 preoral and preanteuual, lying in front of the first pair of auteuure, which it supplies with nerves. 



The brain and ventral nerve cord are now plainly separated from the hypodermis, and are 

 bathed throughout their extent with blood plasma, in which numerous blood corpuscles are seen 

 floating at every point. Giant ganglion cells have become most conspicuous in the- optic region 

 and at the periphery of the brain next the hypodermis. 



The brain is partially divided in front next the optic ganglion by a delicate membrane, which 

 forms' a median superficial partition between its two halves. This is continuous, with a delicate 

 envelope, which in some cases can be detected about the brain, and is like that which covers the 

 optic ganglion and nervous system generally. A similar non-cellular membrane at this time 

 divides the retina from the optic ganglion, and is continuous with the cuticular sheath of the 

 latter. The intercepting retinal membrane is directly continuous with the delicate basement 

 membrane of the hypodermis. The cuticular sheaths of the nervous system are present in the 

 embryo (Figs. 157, 168 p>:), the larva (Figs. 175, 17G), and the adult. It may not seem easy to 

 harmonize this account with the view already taken that the wandering cells attach themselves 

 to the nervous rudiments and form a delicate investment to them (Figs. 129, 131 mes.}. Such 

 is plainly the fate of some of the wandering cells, but the number of cells is probably too small 

 to form a continuous structure, and it is possible that the delicate membrane secreted by the 

 ectoblast may serve as an accession to that formed by mesoblast. 



With respect to insects, Wheeler (07) concludes that in Doryphora the "outer neurilemma" 

 is ectodermic rather than mesoderniic in origin, since 



Shortly after the separation ot'thc IKTVO cord from the integumentary ectoderm, it sheds from its surface a deli- 

 cate chiteuous cuticle simultaneously with the shedding of the first integumentary cuticle. This r.ntirlr, which is 

 separated from the surface of the outer neurilemma, and even from the surfaces of the main neural trunks, is after- 

 wards absorhed. 



At the time when the nervous system has completely separated from the integument there is a 

 slight ingrowth of ectoderm cells along the in id ventral line, most pronounced between the ganglia, 

 and the appearance of a corresponding constriction on the side next the yolk. In transverse sec- 

 tion the jierve cord is somewhat hourglass-shaped. This may be due to a mechanical necessity, 

 arising from a more rapid development of the nerve cells in the lateral masses than in the other 

 parts. 



Ectoblast cells derived from the integument appear to be infolded between ganglia (sec Fig. 

 157 a, thin sheet of cells, with spindle shaped nuclei bending in between the last thoracic and 

 first abdominal ganglion, in the lower right, hand portion of the figure), but these infolilings may 

 be somewhat deceitful, since they are straightened, to some extent at least, with the growth of 

 the abdomen (Fig. 168, abg. I). On the ventral surface of the thoracic region (Fig. IC.s) spindle- 

 shaped nuclei are seen wedged between thi> nerve cords on the middle line. It is not, however, 

 certain that these cells are ectoblastie, since the sternal blood sinus is already formed, to which 



