68 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



mesoderm arises later on, and even during the blastula stage they 

 divide to produce mesodermic cells which partly block up the 

 blastoccele (F, m.s). 



2. Gastrulation. This is effected on the third day by a modi- 

 fication of what is known in other more typical cases as emboly 

 or embolic invagination. This is a sort of inpushing, the nature 

 of which may be realised by taking one of the perforated india- 

 rubber balls used by small boys as squirts, and collapsing it so as 

 to form a double-walled cup. The earthworm blastula first of all 

 flattens (F), and becomes an oval plate, with an upper layer of 

 small cells (ec) constituting the ectoderm (epiblast) and a lower 

 layer of clear columnar cells (en) constituting the endoderm 

 (hypoblast). Mesoderm (mesoblast) is also present (ms). The 

 plate next becomes concave below, and its edges approach. 

 The embryo is now a gastrula (G, H, K) with an internal diges- 

 tive cavity, the arclienteron (ar), opening by a longitudinal ventral 

 slit, the blastopore. The embryo is elongated in the direction 

 which correspond to the long axis of the future worm, and the 

 two mesoblasts (M) are at its posterior end. In fact, bilateral 

 symmetry is well established. 



Fig. 20. EARTHWORM DEVELOPMENT (after Wilson). Enlarged to various 

 scales. A, Unsegmented ovum, surrounded by the vitelline membrane, 

 with two cell-groups formed by division of polar cells. B, First 

 cleavage (in the plane of the polar cells). C, Six-celled stage. D, 

 Optical section of young blastula. E, Surface view of young blastula, 

 showing the two mesoblasts. F, Optical section of a flattening blastula, 

 seen from the side. G, Ventral view of early gastrula (anterior end 

 directed upwards), with wide blastopore. H, Similar view of rather 

 older gastrula, with slit-like blastopore. K, Left lateral view (in optical 

 section) of established gastrula, in which mesoderm bands have met 

 above mouth. L, Right lateral view (partly in optical section) of 

 embryo with established germinal bands and fore-gut. M, Ventral 

 view of same embryo. N, Transverse (slightly oblique) section of rather 

 younger embryo, cutting through germinal bands, the left neuroblast, 

 and one of the right nephroblasts. O, Right lateral view (in optical 

 section) of older living embryo, showing fore-gut, mid-gut, septa, 

 sections of body-cavity, and cerebral ganglia. P, Left lateral view of 

 embryo in middle of development, showing segments, prostomium, 

 head-cavity, nerve-ring and cord, setae, nephridia, and lateral vessel. 

 ar, Archenteron; c.g, cerebral ganglia; cce, ccelotn; d, septum; ec, 

 ectoderm; en, endoderm; l.v, lateral vessel; M, primary mesoblast; 

 m, mouth; ms, mesoderm (upper ms in O = migratory mesoderm budded 

 off from main band); N, nephroblast; Nb, neuroblast; n. c, neural cord 

 (nerve-cord in P) ; np, nephridia; np.c, nephric cords; ps, prostomium ; 

 S.c, segmentation cavity; st, fore-gut (stomodaBum). 



