Table 1 Early embryonic developmental stage of Heterobranchus bidorsalis at 28.5 ± 0.5°C

Zygote

0

Concentration of yolk at the centre; evident jelly coat; increase in perivitelline space (Figure 2A).

Animal and vegetal poles

45

Formation of embryonic disc; distinct vegetal pole; pigmentation of animal pole (Figure 2B).

One-cell

57

Bulging of protoplasm at animal pole (Figure 2C).

Two-cell

69

First mitotic cleavage; discoidal meroblastic division into two equal size blastomeres (Figure 2D).

Four-cell

84

Second mitotic cleavage; meridional division at right angles; 2 × 2 array of blastomeres (Figure 2E).

Eight-cell

94

Cleavage of earlier four-cell blastomeres; division similar to the first mitotic cleavage resulting into two parallel rows of four blastomeres each; 2 × 4 array (Figure 2F).

Sixteen-cell

104

Formation of sixteen blastomeres; 4 × 4 array (Figure 2G).

Thirty-two cell

114

Formation of thirty-two blastomeres; 4 × 8 array (Figure 2H).

Sixty-four cell

127

Irregular cell divisions; overlapping of subsequent cleavages; 2 × 4 × 8 array (Figure 2I).

128 cell

137

Irregular cleavages produce unequal blastomeres of reduced sizes, leading to morula stage (Figure 2J).

Morula

161

Irregular cell divisions; numerous blastomeres appearing like ‘mulberry’ (Figure 2J).

Blastula

165

Flattening of blastodermal cells resulting into two distinct layers, viz: high and low blastula (Figure 2K and L); formation of enveloping and yolk syncytial layers; evident blastoderm (Figure 2M).

Gastrulation

237 (3 h 57 min)

Expansion of blastoderm; epiboly commenced (Figure 2N – 2Q); formation of germ ring; embryonic bud discernable (Figure 2O); thickening of embryonic shield and perivitelline capsule (Figure 2P); increase in foul and offensive odour.

Late (completed) = 668 (3 h 57 min)

Neurulation

669

Neural plate discerned as thickened structure; formation of neural groove delineating the neural keel (Figure 2T); notochord development (Figure 2U).

Segmentation

Early = (1st block observed at) 674

Formation of somite blocks; body pigmentation developed dorso-cephalocaudally (Figure 2U and V). Formation of cranial, dorsal, ventral and caudal regions (Figure 2W).

Late = (completed with the formation of other body organs/organelles) = 979

Early organogenesis.

Optic/auditory buds/Otic placode

Development of otic placode into otic vesicles depicting two tiny otoliths (Figure 2W); differentiation of notochords.

Pharygula period

Development of somites to myotome blocks; formation of sclerotomes; well differentiation of notochord depicting pharygula stage; somites matured (Figure 2U and W).

Caudal bud

Pigmentation continues; somite blocks are more closely packed and well aligned (Figure 2W).

Muscular contraction

1042

Caudal-locomotion commenced at average of 42 s.

Heart-beat

1243

Heart beat commenced at average of 72 beats/min; pigmentation of the point of the heart, eye and other parts increased (Figure 2V and W).

Hatching

1278 (21 h)

Muscular contractions of myotome blocks increased; lashing of the detached and free caudal end against chorion; successful hatching (Figure 2V and W); unique emergence of the newly hatched larvae from the hollowed chorion (Figure 2X).