the seventh cleavage, the SI line totals 64 cells, the M 

 line 8, the St line 4, the E line 4, C line 8, D line 4 and 

 P line (S5 plus P5 or P4 / and P4 II) 2 cells, giving a 

 total of 94 cells. Apparently the eighth cleavage is lim- 

 ited to the SI group at this time. Subsequently a ninth 

 cleavage and at least a partial tenth cleavage takes place. 



Before becoming elongated the SI group probably is 

 composed of 248 cells. At this time there is a rest from 

 cleavage in the SI group during which the other groups 

 (C, D and E) evidently pass through at least some of 

 the cleavages which they have missed, for the mature 

 larva ready to hatch is composed of between 400 and 

 500 cells (Martini, 1907). 



The posterior extremity of the embryo (Fig. 152 EE- 

 HH) begins to bend ventrally and anteriorly at which 

 time the esophagus is well formed, the intestine com- 

 posed of two rows of seven cells; there are lateral meso- 

 dermal chords and a pair of ventral mesodermal chords; 

 the genital primordium is ventral to the intestine and 

 the two cells lie beside one another. The ectodermal 

 cells forming the dorsal and anterior parts of the 

 embryo are larger than the others. 



During this period of elongation (Fig. 153 D-I & RR) 

 several changes take place. The first two intestinal cells 

 divide longitudinally forming- a lumen surrounded by 

 four cells. At this time the genital primordium lies 

 under the sixth and seventh entodermal cells. At hatch- 

 ing the intestine is composed of 20 cells, four at the 

 base of the esophagus and two rows of eight cells behind 

 them. The lumen is zigzag but it later becomes wavy. 

 The cells are in two more or less dorsal and ventral 

 rows. At this time according to Martini (1907) the 

 gonad is situated between the twelfth and thirteenth 

 intestinal cells and is composed of about 10 cells, (Fig. 

 153 D). 



Further differentiation of the epithelium takes place 

 during the same period. Whereas before elongation the 

 embryo is surrounded by two subdorsal, two dorsolateral, 

 and two lateral to ventrolateral rows of large cells and 

 numerous small ventral cells, a distinct rearrangement 

 now takes place. The subdorsal rows which are at first 

 opposite come to be alternate (Fig. 152 EE). (It 

 should be noted that subsequent lettering of cells has 

 no correlation with the lettering used to refer to cells 

 during the first seven cleavages). There is a gradual 

 pushing of the subventral rows squeezing some of the 

 ventral small cells into the body cavity. Slightly later 

 the embryo takes a vermiform appearance commonly 

 called the "tadpole stage" (Fig. 152 GG). By this time 

 the dorsal row of cells is split; it extends from what 

 is now the swollen region, corresponding roughly to the 

 position of the nerve ring, to slightly anterior to the 

 level of the anus. The embryo is left without nuclei 

 in the dorsal line in this whole region. The lateral meso- 

 dermal chords (Fig. 153 G) become dorsosubmedian and 

 form the submedian muscle fields. Some of the cells 

 previously ventral form the subventral muscle fields. 

 These mesodermal tissues pressing against the epithelial 

 cells in the submedian areas cause the six rows of pre- 

 viously mentioned large ectodermal cells to be pressed 

 laterally forming the lateral chords. In the stage shown 

 in Fig. 152 GG there is an anterior mediodorsal row of 

 seven cells (derived from S3 group) which do not separ- 

 ate (d 14-20) but remain as the nuclei of the dorsal 

 chord; the next posterior-most cell, d 13, goes to the right, 

 d 12 to the left and so on. They form the dorsolateral 

 parts of the lateral chords. Two small epithelial cells, 

 6 and B are covered by d 10. Posterior to the twenty- 

 second dorsal cell, d-1 and g o, G o, l-l, and L-l, there are 

 four unpaired cells. The cells destined to form the lateral 

 cell rows of the lateral chords (11,2 etc. of Fig. 152 GG) 

 number eleven on each side, I 7-10 being in the cephalic 

 region, / 1-6 being in the mid region, and l-l, postanal. 

 The cells destined to form the ventrolateral part of the 

 lateral chords (g 0-10) also number eleven, g 8-10 

 forming subventral rows in the cephalic region while 

 the other cells are already in final position. The anus 

 is posterior to g 1 and G 1. The greater part of the 

 nerve cells and the cells forming the ventral chord 

 come from the small cells on the ventral surface of the 

 embryo. 



At hatching the dorsal chord has a single row of 

 nuclei confined to the cephalic region, in which region 

 the lateral chords also have a single row of nuclei, the 

 ventral, two rows. Posterior to the cephalic region the 



lateral chords have three rows of nuclei each, the ventral 

 a questionable number. 



Camallanus lacustris. (Fig. 152 & 153). The embry- 

 ology of this form described under the name Cucullanus 

 elcgans has been studied by Biitschli (1875) and Martini 

 (H03, 1906). There are several minor variations in 

 the form of early cleavage from that seen in the pre- 

 viously studied forms and the development has been 

 followed somewhat further. 



The first cleavage forming Si and PI is very unequal 

 (Fig. 152 P). The four-cell stage is rhomboid at its 

 formation as also in Rhabdias. All of the following 

 cleavages are characterized by smaller furrows than in 

 previous forms and no blastocoele is developed. Subse- 

 quent cleavages (Fig. 152 Q-S) are similar to those in 

 Turbatrix aceti and will be omitted up to the initiation 

 of the ninth cleavage. 



The ninth cleavage of the SI group forms 256 cells, 

 ninth of the C group 32 cells, ninth of the St group 32, 

 eighth M 16 cells, seventh of D 8 cells which, together 

 with the 8 cells of the E group and 2 of the PU group, 

 forms an embryo of 354 cells. This represents a gastrula 

 (Fig. 152 V) the rim of which is formed by several 

 rows made up of the St and M groups anteriad, and by 

 the D and C groups laterad and posteriad. 



Following the 354-cell stage the St cell group divides 

 (10th cleavage) forming 64 cells, the M groups divide 

 forming 32 cells (9th cleavage). The E group divides 

 forming 16 cells (7th cleavage), the C cell groups divide 

 in part forming about 48 cells (9th cleavage), the D 

 cell groups divide to form 16 cells (8th cleavage), and 

 the primordial germ cell P4 divides. It is said that a 

 part of the Si cell group may also divide but this is 

 uncertain. The resulting embryo consists of approximate- 

 ly 486 cells. 



Gastrulation occurs between the 354 and 486 cell 

 stages. The dorsal surface is convex, the ventral sur- 

 face concave; in the anterior part of the embryo the 

 curve is most pronounced in the median line (Fig. 152 W) 

 while in the posterior part it is most marked toward 

 the edges (Fig. 152 X). This becomes more outspoken 

 with age and is correlated with swelling of the dorsal 

 ectodermal cell rows. At this time the dorsal surface 

 of the embryo is covered by 6 longitudinal rows of large 

 cells, derivatives of Si and C and the sides, anterior 

 and posterior ends are covered by smaller cells derived 

 from the same cell groups. (Fig. 152 Y-AA). Gradually 

 these 6 dorsal cell rows come to cover the smaller ventral 

 cell rows which themselves cover the M, St, and E cell 

 groups. The gastrulation is thus through epiboly. The 

 St. and M cell groups are pushed in the groove becoming 

 closed at the anterior end, and there are some cells of 

 the S 1 group which enter the inside of the embryo 

 at the anterior end. At the stage represented in figure 

 152 Z, the posterior part of the ventral groove is open. 

 Finally closure of the posterior part of the ventral 

 groove takes place, the large dorsal cell rows coming to 

 surround the small ventral cells of the D cell group and 

 some of the C cell group. (Fig. 152 CC). At the same 

 time the two most dorsal cell rows fuse so that the 

 embryo is covered by 5 cell rows. 



Organogenesis. It has not been possible to follow 

 the history of individual cells during their rearrange- 

 ment at the completion of gastrulation. Because of this, 

 a new nomenclature is adopted to mark the shapes of 

 further development. The embryo apparently does not 

 increase in number of cells but the cells become differ- 

 entiated into organs. The embryo which is elongate 

 or sausage shaped is covered by a mediodorsal cell row 

 (d 1-20 and s 1-4), 2 lateral row's (I and L -1-10), and 

 2 subventral rows {g and G 0-10). All of these large 

 cell rows are probably derived from C. The "0" desig- 

 nates the position or level of the future proctodeum (Fig. 

 152 CC). The ventral and anterior small cells of the 

 SI cell group, the ventral small cells of the S3 cell 

 group, the E. St, and M cell groups as well as the de- 

 scendants of P4 are all enclosed by the epithelium formed 

 by the above mentioned cell rows. The anterior end of 

 the embryo is covered by five cells, d 20, I 10, L 10, 

 g 10, and G 10. The primordia of all organs are definitely 

 recognizable. 



Formation of the dorsal, ventral, and lateral chords 

 takes place in the following manner. The mesodermal 

 strands push against the outer cell layers in the four 



221 



