258 GASTRULA-ARRESTED EMBRYOS 



H14, after which no further synthesis occurs (Fig. 15). There- 

 fore, according to them, the faikire of hybrids to gastrulate is not 

 a consequence of subnormal synthesis of DNA. But it should be 

 noted that the analytical method employed by Gregg and L0v- 

 trup did not distinguish between free deoxyribosides and those 

 incorporated in nucleic acids. For this reason, it may be best to 

 regard their data as showing only that the total deoxyriboside 

 content of hybrid embryos is not lower than that of controls until 

 after Stage H14. Whether hybrid DNA synthesis is normal still 

 remains to be demonstrated. In this connection, A. B. C. Moore 

 is in possession of microspectrophotometric data (unpublished) 

 which she regards as showing that as long as hybrid embryos are 

 alive their nuclei each contains a normal amount of DNA. 



Summary 



The general picture of hybrid development emerging from the 

 work we have reviewed may now be summarized and commented 

 upon briefly. 



1. At all stages precedent to Stage HIO, hybrid embryos exhibit 

 no developmental abnormalities. 



2. Hybrid embryos are tardy in forming a dorsal lip, but finally 

 do so in the normal manner at late Stage HIO or early Stage Hll. 

 Epiboly seems to begin at the normal time and progresses to al- 

 most the normal final extent, but at a subnormal rate. Archenteron 

 formation gets underway at a slightly lower than normal speed, 

 but is brought to a standstill while it is only one-quarter com- 

 plete. Chordamesodermal convergent extension does not occur in 

 hybrids. Instead, the chordamesoderm spreads on the substratum 

 provided it by invaginated head endoderm, thus effectively put- 

 ting a stop to further mesodermal invagination. 



3. Hybrid embryos, still looking like early gastrulae, cytolyze 

 when they are in Stage H21-H22. 



4. On the biochemical side, post Stage HIO hybrid embryos are 

 characterized by subnormal rates of oxygen uptake, endogenous 

 carbohydrate utilization, and anaerobic lactic acid production. At 

 all stages up to Stage H12, however, their respiratory quotients 

 are normal; only thereafter are they increasingly hypernormal. It 



