MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 441 



The paiivtl structure of the ectodermal plate is well shown in the antennular ganglion on a 

 ievel with the transverse eoniinissures, or even in front of this, where i)aired masses, with small, 

 vleeply dyed niicU'l, are separated by a median slieet of nnicli larger and clearer cells. This may 

 l)()ssibly corresiiond to the mittelstrang, referred to again. 



Shortly after this (Fig. 130) tlie ganglia are blo(!ked off by a series of snperficial constrictions. 

 At least seventi-en sneh ganglionic segments can be connted, beginning with the oi)tic and snpra- 

 a'sophageal ganglia and passing to the last abdominal segments. The ganglionic blocks are 

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

 spicuous i)art of tiie nervous system, unless we accept the large optic ganglia. There is a broad, 

 transver.se, fibrous commissure in the antennular segment, which is still more i)rominent at a little 

 later iXM'iod (IM. XLVI), when eye pigment is forming. From this cominissurc longitudinal rods 

 extend forwards and unite tlie brain witii tlie optic ganglia, wiiih' simihii' I'ods grow l)aekwai(l and 

 form the fibrous axis of the circum-tesophageal commissures. 



The plane of section in Fig. 148 passesjust in front of tlie (esophagus and tiirough the roots 

 of the first pair of antenine (A 1), whicli should appear in the drawing as continuous with tlie 

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

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

 preoral and preantennal, lying in front of the first pair of antenna', which it supplies witli 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, (riant 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, whi(!h in some cases can be detected about the brain, and is like that which covers tlie 

 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 (suticular .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. l~u, 1G8 />»•.), the larva (Figs. 175, 17G), and the adult. It may not seem easy to 

 harmonize tliis account with the view already taken that the watulering 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 proliably 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 (G7) concludes that in Doryphora the "outer neurilemma'' 

 is ectodermic rather than mesodermic in origin, since — 



Shortly atYor tlu> seiiaralioii ot'tlic nerve cord from the integumentary ectoderm, it slieds from its surface a deli- 

 cate chiteuous cuticle simultaneously with the shedding of the tirst integumentary cuticle. This cuticle, which is 

 separated from the siirfiri- nf ti'.' "iili^r neurilemma, and even from the surfaces of the main neural trunks, is after- 

 wards .absorbed. 



At the time when the nervous system has com]>letely separated from the integument there is a 

 slight ingrowth of ectoderm cells along the midventral line, most pronounced between the ganglia, 

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

 tion the nerve 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 (rcUs dcrivt'il trom the integument appear to be infolded between ganglia (see Fig. 

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

 first abdominal ganglion, in the lower right liand i)ortion of the figure), but these infohlings may 

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

 the alxlomen (Fig. I'iS, alt;/. I). On the ventral surface of the thoracic region (Fig. IfiS) si)indle- 

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

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



