410 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



128, Mu.) wliicb extends throughout the abdomen between the hindgut and body wall. Mesobhist 

 cells derived from the yolk (Fig. 129. Mu.) are also seen underneath the epiblast ou either side 

 of the body. The nuclei of these cells stain very deeply, and the cell protoplasm is prolonged 

 into a short fiber or forms an irregular body. In the latter case the cells have the appearance of 

 blood corpuscles. 



The heart originates in the mesoblast (Fig. 135, 7lfe.9.), between the entoblast and outer wall 

 of the body, just behind the thoracic-abdominal flexure. 



At this time fibrous substance (punktsubstanz) is first seen in the brain. It appears as two 

 small masses joined by a transverse commissure, in a plane just anterior to the roots of the first 

 anteuniB. It is distinctly flhrous and apparently originates from the protoplasm of surrounding cells. 



Stage viii. — Segtmentation of the nervous system — at least eight pairs of appen- 

 dages PRESENT. 



We have two longitudinal sections (Figs. 138, 139) to illustrate this stage. If we compare 

 the latter figure with the corresponding one of the previous stage (Fig. 131) we see at a glance, 

 that a long step forward has been taken in development. Between these, 1 have obtained cue 

 intermediate phase, which is a trifle older than the embryo given in PI. XLiv, and can be best 

 described by showing in what respects it differs from it. 



The rudiment from which the nervous system is formed (Stage vii) is a plate of cells extend- 

 ing from the optic lobes to the apex of the abdomen. Anteriorly in the brain (Fig. 132) it is very 

 thick, but gradually thins out as it is followed backward, until it consists of a single layer of epi- 

 blast at the very ti]) of the abdomen. In the phase intermediate between Stages vii and viii the 

 portion of nervous thickening between the oesophagus, which passes through it, and the thoracic- 

 abdominal fold is partially segmented into cell masses, the primitive ganglia. 



The cells of this ectoblastic thickening may be roughly divided into a surface layer, the nuclei 

 of which are large and contain difluse granules of chromatin, and a deep cell mass with smaller nuclei 

 which stain more intensely. Similarly in the optic lobe we find a thick pad of uniform, deeply 

 stained cells (gangliogen) next the yolk, and separated from this a well-marked surface layer (reti- 

 uogeu) of larger ceils. The superficial, epiblastic cells on the inner or ventral side of the thoracic- 

 abdominal fold are large and columnar. The nuclei are very much elongated and closely crowded 

 together, and lie at all levels. This implies rapid cell division in this layer in a plane perpen- 

 dicular to the surface, and as a result of this, the thickening of the ectoblastic plate in this region, 

 such as we see in the next phase (Fig. 139). 



Near the apex of the abdomen there is a transverse zone of very large cells, and the smaller 

 superficial cells adjoining it are arranged in parallel lines. Something resembling this was noticed 

 in Stage VI (PI. XLii, Fig. 120, B. Z.). It corresponds to the budding zone (Knospungszone) which 

 Reichenbach figures and describes in the crayfish. He detects it in a very early stage (Stage B, 

 embryo with rudiments of the mandibles, corresponding nearly to Stage V of this paper), and finds 

 that it consists of a transverse zone of cells containing large nuclei at the extremity of the abdo- 

 men, below the notch of the telson. From it the segments following the mandibular segment are 

 gradually budded off. 



TKe present stage is characterized by the segmentation of the nervous system and the great 

 development of the optic lobes. The ventral nerve plate is blocked out by lateral constrictions 

 into its component ganglia. There is also a median vertical ingrowth or constriction wliich tends to 

 divide the plate into a double cord. It is, however, discontinuous at the middle of each block, so that 

 the ventral nervous system consists of a double chain of ganglia, each p.iir of which are united by a 

 transverse commissure or band of cells, and each ganglion is similarly joined to the one behind or 

 in front of it. Distinct commissures pass from the brain aroniid the oesophagus and unite the 

 former to the ventKil nerve chain. The first pair of post-oral ganglia contain two masses of fibrous 

 substance united by transverse fibers as in the brain. The ganglia following these also contain 

 punktsubstanz. It is developed as a small isolated mass on the dorsal side of each ganglion, 

 toward the middle line. As development proceeds these masses increase in size and are grad- 

 ually united by transverse commissures in each pair of ganglia (PI. xlvi, Figs. 150, 151, Fk.). 



A mass of fibrous or granular substance appears in ciich optic lobe in the gangliogen next the 

 brain. Fibers pass from it to the punktsubstanz of ilie brain, which sends fibers down to the 



