412 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Fig. 146, shows the complete history of development of the retinal layer from its one-celled con- 

 dition toward the median line (already seen at an earlier stage, Fig. 136, Stage vii) to the point 

 farthest from the middle line, where pigment is formed. The fibrous nerve tissue of the gan- 

 liogen now consists of three masses, a ball nearest the brain, which is the first to appear, and two 

 smaller masses between it and the retiuogen. Huge ganglion cells (go.) are of frequent occur- 

 rence, especially at the surface of the eye stalk next the brain. (The details of the development 

 of the eye are reserved for a special section.) 



The brain at this time (Figs. lid-l-iS) differs from that of the previous stage chiefly in point 

 of size. It is composed of nerve cells and large ganglion cells {gc), which occur chiefly near the 

 outer surface, and central fibrous tissue. It rests against the food yolk, and in the living embryo 

 it is bathed with a current of blood (B. S.). The fibrous tissue of the brain has the form of a letter 

 H with a wide and short transverse bar. In front of the transverse commissure (Fig. 147, Tc.) 

 the fibrous substance is prolonged on either side into the optic lobes; behind, it extends down to 

 the ventral nerve cord, on the inner side of the oesophageal ring (Fig. 148, ocm.). 



The ventral nervous system, like the brain, is bathed with blood, which fills a large sinus 

 between it and the yolk. This communicates with extensive blood sinuses extending along the 

 sides of the body (Figs. 148-154, B. S.). In some cases the food yolk, usually in an altered or finely 

 divided state, is in close contact with the nerve chain (Fig. 157). Cells extremely flattened and 

 spiiidlesliaped in section, are found in small numbers closely applied to the nervous system (Figs. 

 152, 151, pr.), and forming a rudimentary perineurium. In most cases they are undoubtedly iso- 

 lated cells, and do not constitute a membrane. They originate from the wandering cells and 

 correspoinl to cells similar in shape and origin which appear between the yolk and nervous system 

 at a much earlier period (Stage Vll, Fig. 131, jl/e.s.). The brain and ventral cord are not as yet 

 differentiated from the superficial epiblast, but anteriorly, flattened epiblastic cells begin to appear 

 between the nerve cells and cuticle. Ganglion cells (Fig. 150) also make their appearance in the ■ 

 cord, commonly at either side, close to the surface. 



There is direct continuity of fibrous substance in the optic lobes, the brain, and the ventral cord 

 as far back as the abdominal ganglia. In the latter this substance has not been develoi>ed. In each 

 siugle ganglion there is a ball of this tissue wliicli is united to its fellow in the same segment by a 

 transverse commissure (Fig. 151), and to the preceding and following ganglion by longitudinal com- 

 missures. It is as a rule completely inclosed by the ganglion cells, but is separated from the yolk 

 or blood sinus often by a unicellular layer (Fig. 150), and in the antenuular and antenual segments of 

 the brain (Figs. 147, 148) the cells next tiie yolk are discontinuous. In the circumcesophageal cords 

 the fibrous tissue also is without a cellular cortex on its inner or central side (Fig. 149,/s.). With 

 slight changes these relations are maintained in the hatched larva (see PI., lv.. Figs. 220-2M2). 

 The foregut is at this time a tube with definite walls and wide lumen (Figs. 148, 152, /(/.). At 

 abour its middle it is bent abruptly backward on itsel£in an acute angle. The first portion, lead- 

 ing from the mouth to the angle, is the oesophagus and is directed forward ; the hinder blind end of 

 the tube lies in nearly a horizontal plane, and represents the masticatory stomach. The walls of 

 the foregut consist of a single layer of columnar cells with large nuclei. They end abruptly next 

 the yolk, but the cavity of the tube is screened from the latter by a thin membrane of flattened 

 cells. A sheet of elongated or spindle-shaped cells surrounds the wall of the foregut and extends 

 over the nervous system (Figs. 148, 149, Men.), while just below the oesophagus and behind the 

 mouth a bundle of elongated cells grows over the nerve cords to the roots of the mandibles repre- 

 senting the adductor muscles, which become so prominent in later stages. These relations are 

 easily deciphered in the structure of an earlier embryo (Stage vii. Fig. 134), where we have already 

 seen muscle and connective tissue elements, derived from the wandering cells, extending from 

 the oesophagus to the epiblast of the body wall. 



Wandering cells also pass into the blood sinuses from all parts of the egg, and become 

 converted into blood cells and connective tissue elements. In Fig. 148 we see two such cells on 

 the edge of the blood stream, and in Fig. 152, (yv.) several of these bodies are in contact with 

 the blood current on the side of the egg opposite the embryo. The relations of the epiblast, wan- 

 dering cells and blood corpuscles in this part of the egg are seen in detail in Fig. 161, where a siugle 

 large wandering cell is already half submerged in the blood. The various other drawings illus- 

 trating this stage (Figs. 154, 155, 160, yc.) point to a similar fate for a portion of the wandering 



