P1130029

1.3.2.3 Day 21 to 40 of pregnancy

On about Day 21 thc bmhryo is detecłabie for the tirst «me near the floor of thc yesicle (GlKTHER 1983 b, K.\hn and Lhix 1984). li is initially v»siblc tis a hyper-echote spot on thc floor of thc embryonic vesieie and not yet separated from the cndometriurn (Fig, 1.58 and 139).

Prom Day 21 to 40 the embtyo follows a charucter-jstiC ascent within its veskle (V>\t.ON et al. 1982). Vcn-traitothc cmbtyo thc allantois starts to ftU up. lifting the embryu (Fig. 1 ,61 to 1.66), Włute the yolk sac above the embtyo initially oceupied the bulk of the carly coneep-tus it nóvv shrinks over thc next few days. The rado be-twcen the sizes of the two embryonic structures grudu-aliy moves tu favor of the allantois unit! thc yolk sac hus siitually disappeared by Day 40. The embryo appears to be suspended by the hyperechoic membranę formed by allantois and yolk sac and floats insidc the ancchoic em* btyonic fluid. The majority of embryos commcncc their asceot between Days 22 and 25. On Day 27 they are still suspended in the bonom third of the vesiclc (Fig 1.62).

by Day 30 ifl thc center (Fig. 1.63) and In Day 33 m thc top tlurd of the yesicle (Hg. 1.64 and l .65).

On Day 25 thc embtyo is about 5 mm ioiig (OłN* tut k 1986). It grows by ubdut l mmpcrdaytoreaeh 12 mm by Day 30,17 mm by Day 35 and 22 nurt by about Day 40.

The embryos hcart beat can be dctected annmd day 26 whieh is only a few days after the embryo can bc first dctected. The hcart beat is an important critcrion for thc assessment of embryonic viubility and for the cxdu* sion of a pathologieal pregnancy.

After thc suspended growth period of the conccptus from Day 19 unlil Day 25 therc is a renewed growth (Fig. 1.55). The growth ratę until Day 50 of gestation ts about 2 mm per day. From thc fourth week of gestation a differenee in thc growth ratę betwcen fetuses of differ-ent breeds becomes evidcnt; hence the diameter of thc embryonic veside of heavy draught horses betwcen Days 20 and 40 of gestation will be I to 4 mm larger than that observed in iighter horse breeds (CHEvai iik and Palmir 1982).